Looks good, but, usability wise, we probably need some kind of =default for it.
Any task type that does not do anything special for await_transform would need to lookup and paste the boilerplate for await_value. Source of bugs.

Can the = default definition be added later or should we target that for this change too?

Sure thing. I am not requesting a change. Just just expressing my "feelings" about this change. :-)
Coroutine authors have to memorize this sequence by heart.

"template U&& await_value(U&& value) { return static_cast<U&&>(value); } "

Note that in the p1477r0 detached task type forgot to include the await_value in its promise definition :-p

I think it could be challenging to allow = default for an otherwise normal method. Maybe if it was named operator co_await() then it would be more palatable.