Some here are saying "Why not just use Rust/D/Zig/Nim/Crystal if you're going to break backwards compatibility?" I believe the proposal is closer to "We've removed this old feature, run this tool to replace it with the new one." C++ will keep looking like C++, not like Rust. Here's Google's "Codebase Cultivator" explaining the idea for their Abseil library:
https://youtu.be/tISy7EJQPzI?t=2209
I remember watching another talk where they propose something similar for the language itself, but I can't find it at the moment.