"The useful addition might become deprecated in a few years" is a poor excuse considering that all the time niche language features are being added which either take forever to implement, or end up getting banned in real codebases.

How is it okay that newly added language features end up unused/deprecated/discouraged, but it's a problem for libraries?

I never got to try exceptions and modules in C++.

Exceptions because every place I worked at banned them, and modules because they were not implemented by the time I left the language.

I'm 100% sure that people got (and will keep getting) much more value out of unfortunate std::regex and std::fstream than they will ever get from reflection (in the few places that will even allow using it at all).