Given the traction safe languages like rust, C#, typescript/javascript, swift, etc. have, I suspect the time for this is long past -- if there ever was a time.

I think the problem is a "safe C++" might look a lot like C++ but will have enough fundamental differences to be like a new language from a practical standpoint... devs will have to learn and use new patterns, new tools. Old code will need to be ported and possibly significantly rewritten, since we're talking about memory and threads. At that point, you might find using one of these other safe languages a better option.