What's the reason reason/need to have this exception? Is this just for development (perhaps a local patch would be good enough then?), or to make it easier for people to try simple things out (but does a nested server provide the ideal environment for that?), or are there other use cases?
There isn't a fundamental reason that's preventing the implementation of these protocols in nested mode, so it seems to me that making them optional introduces an exception for just a couple of compositors that haven't implemented it for nested mode (yet!), which I don't think is the direction we want to move towards (note that all major compositors support the protocols in non-nested mode).
In the end, the change is small enough that I don't mind it either if it's useful for you, but it's not something I would propose myself.