I think that testify is critical to the Go ecosystem. We must not block users from upgrading testify if they can't upgrade Go (example: removal of a GOARCH).
io/ioutil will stay forever (because of the Go 1 compatibility promise) so there is no immediate need for dropping its use.

So I think we must block this for now.

We already have this problem by using Go modules (1.13), as well as specifying a minimum Go version in the go.mod as 1.19.

I haven't done an exhaustive search, but I'm guessing there are already features in use that make this unusable in older versions.

The CI tests are updated, so a rebase should show this is safe.