Why should this be in the standard library? https://go.dev/doc/faq#x_in_std
What are the semantics of a blocking callback?

https://go.dev/doc/faq#x_in_std

not sure how it answers my proposal

What are the semantics of a blocking callback?

this callback can be running in a new goroutine, so it won't be blocked. it could be a simple func() with no return value. The main idea is to be able to trigger on canceling without keeping the running goroutine for checking the context state.

for downvoters: read this proposal as "event driven" vs "polling" approach.

<-ctx.Done() is also event driven and not polling.

<-ctx.Done() is also event driven and not polling.

sorry, but it has nothing in common with "event driven". its literally "polling" approach

Given that we currently have a canonical way to do this, and it hasn't been demonstrated to be a widespread need (eg by third party libraries implementing custom contexts with this feature), I think https://go.dev/doc/faq#x_in_std is the best answer.