a. It has been adopted by Mastodon and Alt-Right SNS. It seems that the purpose is to prevent reflexive Reposting.
b. Mastodon and Farcaster cannot. Also, Nostr is divided into clients who can do it and clients who can't.
c. It is available for Misskey, Firefish, Pleroma, and Akkoma. Nostr is also divided into available clients and non-available clients.
Also, I think this is a feature unique to Nostr, which has multiple client apps.
Reposts and likes cannot be canceled.
Amethyst looks like it can be canceled, but
It will remain visible from other client applications.