Right. The only restriction we're looking at is that post-5.0 language and library features that require concrete runtime or library support may not be back-deployable to 5.0.
Obviously, that restriction isn't ideal. For example, I think it would be really unfortunate if you couldn't use
await until you were willing to bump your deployment target to 6.0 or whenever we ship that. It may be possible to ship stubbed-in support for certain features on older operating systems. But that sort of thing is non-trivial to test and support, and we don't want to promise to do it for literally everything added to the language.