[diath]

C++ instead needs language editions, legacy and modern. Let compilers implement a switch for which edition you're targeting. Legacy does not touch backwards incompatibly changes or language features. Modern cleans up everything.

[ceteia]

I don't think that would be a silver bullet nor without drawbacks. It can be a rather bitter footgun when the same code can have different semantics in different editions or epochs. Automatic or semi-automatic conversion, as Rust has, helps it a bit, but not fully.