See the last paragraph of zarzavat's comment :) They wouldn't be added today, and they aren't widely used any more (they have non-codegen alternatives now). In other words, breaking changes in semantics shouldn't be a worry any more, since the TS team has consciously decided to attempt to avoid them a few years ago.
Edit: ok, it says it affects whether the program will typecheck, so it's consistent with not affecting code generation.
(More Turing completeness discussion at https://github.com/Microsoft/TypeScript/issues/14833)
Then there are other features that have no equivalent in JS. For example enums. And the handbook talks about what kind of JS they "compile to".