I think it is nice to have at least one working implementation (good for figuring out all the things one forgets), but there is no strict requirement.
Excellent question.
FEPs do not have any control of what should be added to ActivityStreams. Maybe it might make sense to add a FEP namespace (e.g. https://activitypub.dev/fep/ns#
) where things can be added but in the mean time I think it is better to use your own.