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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.

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.

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

Might I suggest that the technology has been vetted?

vet: to subject to usually expert appraisal or correction

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

As the dictionary example:

—often used figuratively
The Yankees' pen, moreover, is battle-tested. Rivera, and his setup men, lefty Mike Stanton and righties Jeff Nelson and Ramiro Mendoza, have a combined 1.45 ERA over 105 2/3 innings of postseason experience.
— Tom Verducci, Sports Illustrated, 20 Sept. 1999

For your example:

It is rather old, but battle-tested technology.

This would give the impression that the software has been put through its paces under considerable duress, unexpected uses and intense situations - and each time, came out still working!

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.

But it fits your word search nicely nevertheless because it already encompasses the "rather old" aspect. You can simply omit the "It is rather old" and say "It is a seasoned technology."

May I suggest the adjective fully fledged:

Fully fledged means complete or fully developed.

This adjective comes from the idea that when a young bird has acquired its adult feathers, it is able to fly.

So, the OP's original example sentence is going to look like this:

It is rather old, but fully-fledged technology.

unbeatable

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

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

  
  

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

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.

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.

I like mature or proven, but consider refined:

It is rather old, but refined technology.

I believe it gives a stronger connotation of something that was actively improved over time.

1.2 Developed or improved so as to be precise or subtle.
_{Oxford Living Dictionaries}

The strength of a mature or proven technology is the implication that it has been in use for a long time with few faults. It doesn't necessarily mean that it has been actively improved over time.