I don't think either Swift or Kotlin have "optional type annotations" of the sort mentioned in the comment I was replying to...
In this context "optional" means "you can either use or not use type annotations", which is not related to type inference [1] but is a way to gradually add types, usually in a half-baked, unsatisfying way.
[1] because unlike with Kotlin or Swift, there's no inference when you leave out the annotations, but instead you "switch off" static typechecking for portions of the code you don't want to annotate.
Yes I know, but I think we're discussing the extent to which ideas from FP languages leaked into the mainstream? Optional-as-in-dynamic typing didn't come from the FP world, whereas optional-as-in-inferred did.
In this context "optional" means "you can either use or not use type annotations", which is not related to type inference [1] but is a way to gradually add types, usually in a half-baked, unsatisfying way.
[1] because unlike with Kotlin or Swift, there's no inference when you leave out the annotations, but instead you "switch off" static typechecking for portions of the code you don't want to annotate.