If you click on the “more” button under a comment or link there will be an activity tab. In this tab you can see everyone who has boosted, favourited or reduced the post. I’m not sure if this a
Is a good feature but it’s interesting to see when someone decides to reduce all of your content for no reason.
The last bit is not strictly true - if you delete a post/comment it will federate the deletion so it will (should) delete everywhere. Any hosts which are off-line or later defederated might still keep a copy of it though, or a user client may have it cached. “Be careful with what you write” is always good advice regardless!