We have now the replacement (MutationObserver) for mutation events unprefixed. It is significantly faster way to observer mutations. We should consider blocking addons which use old Mutation Events.
We have generally not accepted mutation events on AMO, and we're encouraging all developers to move to the new model. Some still need to use the old system for old versions, and I don't think that's worth blocklisting for. What we can do is figure out who is still using them and give them the information about the new events. I don't think blocklisting is in order here.