Unless you manage to integrate this directly into the compiler stack and have it work 100% of the time with no "ifs" or "buts" I don't think it'll work.

Maybe you could do like Rust with their "Epoch" system that lets you interoperate code using various standards in order to migrate progressively without breaking backward compatibility. I suspect that it would be a lot harder to make it work for C++ however, mainly due to its extreme reliance on #includes (especially for anything using templates) and more common use of macros.

I'm not saying it's impossible but I suspect that it would fragment the ecosystem quite a bit. Removing "old features" tends to have massive side effects in a language like C++ with metaprograming, overloading, multiple inheritance, unlimited macro usage and complex symbol resolution rules.

So I think "why not just use Rust/D/Zig/Nim/Crystal" is warranted feedback for these proposals (and you could probably add Go, C#, Java and a few others).