A lot of languages are going this direction too:

* Cone is putting a full borrow checker on top of GC, RC, and single ownership: https://cone.jondgoodwin.com/

* Vale is putting opt-in "region borrow checking" on top of single ownership with generational references: https://verdagon.dev/blog/zero-cost-refs-regions

* Lobster is employing an automatic-borrow-checker-esque algorithm on top of RC for some brilliant speedups: https://www.strlen.com/lobster/

* Ante has some designs for lifetime inference, which I think will work really well: https://antelang.org/

* HVM is a runtime for Haskell that uses borrow checker semantics and falls back to clone. It's not shared mutability, but Haskell without GC is pretty cool! https://github.com/Kindelia/HVM

I particularly like D's approach because it's opt-in; we can use it in the areas of the program where we need performance the most, and prioritize flexibility and development velocity everywhere else, which I think is the right balance for most programs developed today.

It also means one can gradually ease into learning how to use static analysis and their more complex constructs (like return scope, mentioned in the article). One can be proficient in the basic language quickly, and then improve their craft gradually. In modern software engineering, this is a big win, in my opinion. Props to D for going this direction!