to follow up on the subject line for emails generated by chromestatus.com
Assuming this is an intent to ship, it seems pretty reasonable to me conceptually. That said, it is a deprecation and removal in disguise, as it going to prevent frames that are currently triggering share actions from doing so until their embedders update. That seems like the right long-term outcome, but it would be helpful to quantify the short-term implications. Do we have UseCounters that would help us understand the potential breakage? I'm thinking of embedded services like sharethis (which might or might not actually use the webshare API?), and would be left with broken buttons. Is there data you could provide, perhaps from HTTP Archive, that could set an upper bound on the short-term impact?
You also note that WPT doesn't cover this functionality yet. It would be helpful to add tests to verify the nested behavior.