It is not uncommon that older/senior developers hold an undeserved grudge against STL. STL was far from mature in the 90s, sometimes even rather bad (at least in Windows/Visual Studio). But that is simply not the case anymore. Modern STL is well-written and highly optimized IMHO.
Sure, it is not as "complete" as standard libraries in Python, Go, etc. It is rather a different kind of beast than those standard libraries, not as high-level, more building blocks oriented.
The allocation patterns that the STL requires / encourages, as well as the usability side (error messages) and the compile speeds, probably cannot improve unboundedly, given that they API has to stay the same.
IME the main problem of the C++ stdlib isn't the implementation quality, but the interface design. It must be everything to everybody, but at the same time doesn't provide much control over the internal behaviour. And the interfaces can't be changed because of source code and binary compatibility requirements.
In many (most?) cases, writing your own stdlib alternatives still makes a lot of sense.
From my point of view the STL still suffers from a major flaw versus the compiler frameworks from the 1990's.
The whole team needs to care about secure code to turn on checked iteration in release builds, or write their own wrappers if portability to compilers without such support is a concern.
Sure, it is not as "complete" as standard libraries in Python, Go, etc. It is rather a different kind of beast than those standard libraries, not as high-level, more building blocks oriented.