> what ensures that ‘// used a suboptimal sort because reasons’ is updated during a global refactor that changes the method?

And for that matter, what ensures it is even correct the first time it is written?

(I think this is probably the far more common problem when I'm looking at a bug, newly discovered: the logic was broken on day 1, hasn't changed since; the comment, when there is one, is as wrong as the day it was written.)