> "how to make STL implementation less embarassing".

The "STL" part of the standard library - containers especially but not just that - has an outdated interface, and suffers from ABI being stuck:

https://cor3ntin.github.io/posts/abi/

so it's embarrassing regardless.

> there still is no language support for e.g. debugging constexpr, or printing internal private state of objects in 3rd party code.

Actually, reflection might make it easier to do that. Supposedly, you should be able to get a member pointer to the private member you're interested in (or even do it dynamically by iterating over all members, and figuring out which one you like), from that and an actual object obtain a regular pointer, and finally dereference it.