Basically subsequent C++ compilers provide the possibility of some subset of C++ which is considered modern. Then files can be marked with it, at which point the C++ compilers reject deprecated idioms. Over time this should take over, I think most of the JS in the wild is now using strict mode.
I can't be the first person to think of this. There must be a reason something like this hasn't gotten traction.
Also many of the issues are intrinsic to the way the language has evolved (the template compilation model for example), it is not just a matter of deprecating a few features here and there (although that might help)
There have been plenty of from-scratch languages that are designed as a better C++. Many have died, of those left no one has yet gained significant traction [1].
It is not clear why yet another an incompatible language would be better just because it happens to be called C++. The effort would be better spent on improving one of the existing alternatives.
[1] Plenty of newer languages have had breakaway success, but no language that is meant to be a full C++ replacement has yet. D has passed peak hype without a breakout. Rust might still make it, we will see.
If I had the choice of having a C++2020 with absolutely no backwards compatibility but a greatly cleaned up language or having yet another step down the path it's heading now, I'd chose the former even if it breaks backwards compatibility. But the language is going to follow the developers and they want to continue down the path where they get to explain again and again and again the intricacies and delicacies of rvalue semantics, exotic template meta-programming and what-ever the next big thing is going to be. All the while we seemingly still don't even have a clear road-map for providing simple modules.
I was enthusiastic back when C++11 hit and it felt like a great big push going on in the language development, I had the feeling that things were really going to take off... but now it feels like it just died out, and now there's nothing that really gets me exited about the future of C++. I'm worried It's going to get more and more complex and less and less used.