Claire:ActivityPub defines Undo as undoing side-effects, not removing the activity itself.

[silverpill@socialhub.activitypub.rocks]

Claire:

ActivityPub defines Undo as undoing side-effects, not removing the activity itself. Here, we would want the stamp to disappear.

I agree, activities shouldn't be Delete-ed. However, some implementations may rely on target being reserved to activities. For example, at some point there was a discussion about "duck typing" activities by checking for ( actor and object) and (actor and target) property combinations. I am not aware of any implementations doing this, but I think it would be prudent to use a different property. Perhaps it could be generator (awkward, but its domain and range include Object)? Or you can just add a warning to the FEP to make implementers aware that such use or target is not recommended by AS.