ah sorry I minimized the repro further after my initial post, Just updated that.
... I am shocked to realize you are right, that this is in fact a compiler default - [URL] defaults to undefined, which only shows unreachable code as warnings. However, typescript playground still has no way to actually set that to undefined it seems. So the behavior does differ, just perhaps not usefully.
I still hold that it would be much more useful to include a warning about the code being unreachable when narrowing the types to `never`s. Perhaps that shouldn't be error `TS7027` but rather some sort of extension of the "hey this is never so this doesn't work" error messages, for those of us working without editor integration.