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I don't know of a technology that cannot be improved, but we often use the term mature to describe technology that's deemed developed enough to be left alone:

A mature technology is a technology that has been in use for long enough that most of its initial faults and inherent problems have been removed or reduced by further development. In some contexts, it may also refer to technology that has not seen widespread use, but whose scientific background is well understood.
Wikipedia

mature

6. No longer subject to great expansion or development. Used of an industry, market, or product.
American Heritage® Dictionary

You may be looking for proven.

It is rather old, but proven technology.

Collins:

proven in British

adjective
3. tried; tested

a proven method

Collins English Dictionary. Copyright © Harper Collins Publishers

proven in American

adjective
2. known to be valid, effective, or genuine

a proven method

Webster’s New World College Dictionary, 4th Edition. Copyright © 2010
by Houghton Mifflin Harcourt. All rights reserved.

tried and tested

adjective - recognized as reliable; found to be successful

tried-and-tested in British (ˈtraɪdəndˈtɛstɪd), tried-and-trusted
(ˈtraɪdəndˈtrʌstɪd) or US and Canadian tried-and-true (ˈtraɪdəndˈtruː)
adjective recognized as reliable; found to be successful

https://www.collinsdictionary.com/dictionary/english/tried-and-tested

As noted the expression varies somewhat according to location. All of the above versions would be understandable to a British person.

Most common term I have seen repeatedly and would thus use is, well established.

It is rather old, but well established technology

"science can be leading edge or well established" Wiki Technology

I'd suggest perfected, to capture both the "fully optimized" and "fully reliable" qualities. (Which are not at all the same thing!)

It is rather old, but perfected, technology.

Might I suggest that the technology has been vetted?

vet: to subject to usually expert appraisal or correction

All of these answers, mature, well established, proven are absolutely correct in my opinion, but have become euphemisms for "out dated". Whenever someone tries to sell me "mature" technology, I immediately think "barely usable in today's environment".

I would then describe this as stable:

Not likely to change or fail
; firmly established

or fit for purpose:

well equipped or well suited for its designated role or purpose

The word that first came to mind for me was robust, which a previous poster had used in describing the meaning of the word they were suggesting ("foolproof").

robust

1d : capable of performing without failure under a wide range of
conditions

• robust software

In the same vein as the good tried and true or mature is seasoned. The Merriam-Webster entry gives "to make fit by experience" as one of the meanings of the transitive verb. It is used in expressions like seasoned advice or seasoned veterans or seasoned strategic planning consultants.

I think seasoned implies that something or somebody functions properly, without fault. This likely includes sufficient efficiency but does not necessarily mean strictly optimally, so it's only a partial fit.

I'm thinking perhaps flawless may suit the need.

It is rather old, but
flawless
technology.

• 
  

  flawless
  adjective

  
  
  

  flaw·​less | ˈflȯ-ləs

  
  
  

  Definition of flawless

  
  
  

  1 : having no flaw or imperfection : PERFECT

  
  

Otherwise consider immaculate if you prefer it instead.

It is rather old, but
immaculate
technology.

• 
  

  immaculate
  adjective

  
  
  

  im·​mac·​u·​late | i-ˈma-kyə-lət

  
  
  

  Definition of immaculate

  
  
  

  2 : having or containing no flaw or error

  
  

unbeatable

adjective UK /ʌnˈbiː.tə.bəl/, US /ʌnˈbiː.t̬ə.bəl/.
Unable to be defeated or improved because of excellent quality.

I would offer battle-tested

Often used figuratively to imply its been used hard and been put through its paces and hardships - yet still remained viable.

Battle tested

Streamlined may fit your needs.

It is rather old, but streamlined technology.

adjective - designed or organized to give maximum efficiency; compact.

https://www.dictionary.com/browse/streamlined

I somewhat prefer this since it doesn't speak to the "newness" of the technology but it does imply that it is optimized or researched/planned in detail.

Since the sentence already contains the context of "technology"...

It is rather old, but _____________________ technology.

An adjective like well-developed could work.

Broadly defined, it means highly or fully developed, so it allows for the possibility that nothing is ever fully developed, so to speak (as previously suggested), and also permits the assumption that something may be...like my sixth hammer, obviously; the first five (all being the same brand, style, and weight)--'Misappropriated!'--carted off like Helen of Troy (Who could blame them?).

But hammers, even six of them, probably don't count as "technology"...

So, here's another example of fully developed technology found online, a video.

Well, that is some fine-looking machinery there...but I wonder if the wire-processing people thought their technology was fully developed before the invention of touchscreens. Hmm, it's just a thought.

The phrase that comes to mind immediately for me is a well oiled machine.

Macmillan dictionary notes that the adjective phrase describes something that "operates without problems;" Merriam-Webster uses the defining phrase "smoothly functioning."

The word or the phrase should address a technology, which is totally investigated, researched into detail, thoroughly optimised so there is nothing more to find out or improve.

"Well-oiled" indicates that everything is running optimally with no clear room for improvement and no implied efforts toward further investigation or research, but rather continuing with the great status quo. Also, with more and more machines going digital, use of this phrase would cause a mental association with older technology, but technology that is still functioning well.

an efficiently robust technology

In a technical context, you want this definition of the word understood, which carries more weight than the way it is used colloquially:

un·​der·​stood | ˌən-dər-ˈstu̇d

adjective

1. fully apprehended

The reason is understood is meant to be complete. If it is 100%, totally investigated, researched into detail, thoroughly optimised so there is nothing more to find out or improve, in science it is referred to as understood. Examples include:

These are all interesting mathematical properties that are relevant to the description of physical systems, but they cannot be used as a catch-all, by saying that eventually all (un-understood) physical systems will be understood in terms of non-linearity.

Source

And

On the other hand, it has been found that many effects occuring in natural and man-made system of propagating waves can be best characterized and understood as parametric interactions of waves.

Source

In both the these examples, the understood concept is a fully studied one that can be safely used as a building block for more complicated models because there is nothing more to learn about it.

There may be a place for the term de facto standard here, which states that some (technology) is, in reality, pretty much the only sane choice. This scenario naturally goes hand in hand with established, mature technologies that do the job brilliantly.

The hydraulic caliper disc brake is the de facto standard braking architecture in cars.

Foolproof could be an appropriate choice if the robustness is the concerning aspect of that. On the other side, thinking that as the best instance of its breed, the ultimate is suggested.

There is a common acronym for this, used in computing and engineering, which is

COTS - "commercial off-the-shelf" - i.e. you can buy it ready made without having to do any research to invent it yourself, and without needing to understand every detail of how it works.

It is rather old, but COTS technology.

https://en.wiktionary.org/wiki/COTS

Nobody so far has suggested commodity (noun) / commoditised (verb). Where something is so well-understood that anyone can make a new one cheaply, that won't be any better than the other options.

I work in the development of new technology and we often refer to things that aren't interesting to us any more as having been commoditised.

The Merriam Webster definition of commodity covers it under heading 3:

3: a good or service whose wide availability typically leads to smaller
profit margins and diminishes the importance of factors (such as brand
name) other than price

I think the current top two answers are probably the ones I'd use. However, depending on your context you may prefer the term exhaustive.

It is rather old, but exhaustive technology.

You might have the modify the sentence to add more context for it to make sense since the word exhaustive rarely stands on its own, but to me it conveys the idea that the technology has covered all the bases, so to speak. They've thought of everything.

******thoroughly vetted*******