It makes a lot of intuitive sense to me, in general, to treat features and extensions the same way. I don't think any better designs occur to me.
Of course, we already started down a different path with shaderStorageImageReadWithoutFormat in 3dbd2ceca. I still am not sure that was the best option, but perhaps we should just be consistent...
That, but also whether it's really worth it to change the default value of this between releases. Those aren't entirely independent choices, of course; using the extension mechanism makes it a bit awkward to keep this enabled by default like it was in previous versions.