I added a public constructor for TypeAliasType to my implementation in gh-103763: Implement PEP 695 by JelleZijlstra · Pull Request #103764 · python/cpython · GitHub, as I think it makes sense to allow programmatic creation of these objects if people see a need for it.
Before we backport this in typing-extensions, I’d like to see a discussion on typing-sig though, because it does put a bit of a burden on type checkers to add support for this feature.