I use Emoji Reactions to indicate to others if their messages have been actioned - usually a little green check mark.

If a message is for me specifically to action, I tend to save it and use that saved messages sidebar as a sort of "to-do" list. If I react to a message, that emoji shows up in my saved list and I can then make sure I un-save it.

This behaviour doesn't seem to be replicated in the Recent Mentions view. It would be good if that view included reaction emoji's, so I can quickly glance to see I've actioned all messages that I've been tagged in.