I would remove result builders and all other uses of @attributes that change the semantics of the code (e.g property wrappers).

I would remove the distinction between value types and reference types at the type level. This has caused so many bugs in my code. This distinction should be made where the types are used not where they are defined.

I would remove everything related to concurrency from the language itself. The idea to let code execute on random threads without any explicit hint at the call site is ridiculous. It's far too complicated and error prone, which is why Swift designers had to radically change the defaults between Swift 6.0 and 6.2 and it's still a mess.

I would remove properties that are really functions (and of course property wrappers). I want to see at the call site whether I'm calling a function or accessing a variable.

I would probably remove async/await as well, but this is a broader debate beyond Swift.

And yes you absolutely do have to know and use all features that a language has, especially if it's a corporate language where features are introduced in order to support platform APIs.