> Most static type systems that I know of disappear at runtime. You literally cannot "use" them once deployed to production.

That's part of the point of being static. If we can statically determine properties of the system and use that information in the derived machine code (or byte code or whatever), then we may be able to discard that information at runtime (though there are reasons not to discard it).

> Following OP's example, what prevents you from getting a AccountID parsed as a UserID at runtime, in production? In production it's all UUIDs, undistinguishable from one another.

If you're receiving information from the outside and converting it into data in your system you have to parse and validate it. If the UUID does not correspond to a UserID in your database or whatever, then the attempted conversion should fail. You'd have a guard like this:

  if user_db.contains(UserID(uuid)) {
    return UserID(uuid)
  }
  // signal an error or return a None, zero value, null, etc.