As far as I know, none of the new extensions in 5.10 are enabled unless a switch or pragma is used, so old code can continue untouched.

The point about %{} is that it is a fairly natural extension of existing mechanisms that doesn't conflict with existing uses. That there may exist by happenstance some code that would need to be changed in minor ways before it would run under the auspices of the feature pragma is not, (should not be), a barrier to the addition of new features.