It doesn't even have to stop progress. C++ does a new rev of the standard every 3 years. Rust has editions, which is a similar but less rigorous concept, and which could be made more rigorous.
Just as there is a C++ standard and multiple C++ compilers that implement the standard, there should be a Rust standard and multiple implementations.
This is the way that mature languages work.
If this is the benchmark, then among popular languages you basically have C, C++, C#, JavaScript, and... is that it?
That being said, I do think, considering it longer, there are languages that I am missing, like SQL, and ones that have a spec, even if it’s not under an ECMA/ISO process, like Java and Go, that I was forgetting.
I still think that using this as a necessary condition for “maturity” is misguided.
Specs are definitely a good thing to have, but often they're just brandished as a bullet point without looking at the details: how good is the spec, what does it add over the existing tests/CI/RFCs/proofs, etc.
There is a COBOL specification, but AFAIK nobody actually implements it fully. Implementations pick and choose new features based on customer demand.
Also, COBOL is dominated by large legacy codebases, which means that if there's a discrepancy between an implementation and the specification, the users normally don't want it fixed, because they may have written code that depends on the "incorrect" behaviour and it's a lot of work to audit.
IBM built a new backend for its COBOL compiler using the optimizer from their JVM. IIRC by the time I left it generated code that was ~2x faster than the old compiler, but uptake was still slow because of migration concerns. In particular, we spent a lot of time working on features to help guarantee that code with some forms of undefined behaviour would have the same result as the old version.
Having a webapp depend on a CPython implementation detail is very different from having a kernel depend on an implementation detail of a language without a spec that was used to implement it.
And languages actually stack on top of each other. Imagine depending on a CPython implementation detail that depends on an implementation detail of a specific C compiler. Those things do happen and they make programmers' lives miserable sometimes, but imagine how often that would happen if C didn't have a spec that all compilers strive to implement.
Ada, Fortran, Cobol as well.
This is specially relevant in the industrial sector with certified compilers.