Sometimes, changes to Swift can be limiting on future evolution. For instance, see the pitch to elide commas.
So, should a discussion of the potential effect on future evolution be a standard component of a proposal?
I think, yes. Authors should consider the potential limitations, and discuss the same as part of their proposals. Reviewers should consider the same.
Usually, this discussion happens as a natural part of a proposal review, but it may deserve more emphasis. Making this discussion a standard component of a proposal, perhaps under the heading "Limitation of Future Evolution," would bring it to the fore.