> Note that the ECMAScript decorator spec now supported in TS 5 doesn't support decorating method arguments, unlike the experimental decorators TS has had an that Angular uses e.g. @Inject, @Self, @Optional etc. I'm guessing that this is part of the motivation behind the inject() function, where you can pass flags to it as a second argument to support cases like self or optional - it gives them a path to moving to the new-style decorators in the long-term.
AFAIK argument decorators are only used for constructors, and that usage can be replaced by using inject() with flags, so a migration would be possible even today - although I'm sure there would be plenty of as-yet uncovered subtle implementation quirks!
The obvious headache TS 5 will introduce is that failing to follow a decorator name with () will now attempt to treat e.g. Inject as a new-style decorator and cause a type error of some kind, which is likely to be confusing. I'm guessing the Angular compiler will have to do something to manage this, even if it's just ensuring there's a useful error message.