Thanks for starting this, Jelle - I was a bit unsure about how to proceed here.

Given that https://github.com/python/cpython/pull/31800 is already merged, I'd also propose something halfway between the two extremes: return a sensible substitution when the logic to compute that isn't too onerous, and a new GenericAlias object when it is. The upsides are that we'd probably be able to return reasonable substitutions for the vast majority of cases, and that we wouldn't have to remove what's already been merged. The downsides would be lack of consistency, and the potential for changing rules about what does and doesn't return a full substitution as time goes on and new features are added.