C++ probably needs an "unsafe" keyword like Rust to isolate the non-"good parts". A few hundred compiler flags could be used to distinguish the good and non-good parts. I don't think that 5 years is enough to know all things C++; it's like a black hole in the sense that it adds features faster than you can learn how all the different parts could interact - even if you dedicate your whole life to it. Maintaining old C++ codebases will employ a lot of old developers for many many years. Your employer should not expect you to understand an old code base if you are a new developer. This would even be counter-productive since it wastes your brain power and their money.

What modern C++ really needs is a rebranding. Something like "C22"/"CX"/"Z" to distinguish itself from old C++. Together with a guide on what the safe features and "unsafe"/deprecated features are the developers could get new certificates to prove that they are up to date. Companies/management could easily require that the new C++ is used and developers are actually capable. (This organizational issue is IMHO the most important part.)