I think it’d be better to support “transpilers” that transform new Rust syntax to old Rust syntax, same as Babel does for ES6/2017 to older JavaScript.
It would be less capable (you couldn’t get NLL, unless you reinvent the whole thing and emit a soup of unsafe pointers?), but all syntax-sugar and library features could be backported this way, and it’d be entirely stable, because it’d ship its own compiler.