Tukukkk

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.

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

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.

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

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.

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, instead of 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.

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.

unbeatable

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

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.

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