I use flags to remember what I need to do with a task (for example, read this/test that/etc., then comment/resolve/etc.). Therefore it would be nice if, after I edited/commented on a task that I had "flagged for later", Phabricator would display the flag's note and ask if the flag should be removed now.
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I think it's too hard for us to guess user intent here. There's nothing a user can do which strongly signals that their flag is obsolete, since flags mean a lot of different things (in your use case, it sounds like there's no single action you take which clears the flag, just "the next action after you added the flag"). Other users (myself included) use flags for different reasons, including as long-term bookmarks. Guessing intent is also difficult because you can flag many kinds of objects, not just tasks, and we'd strongly prefer features which can be implemented globally.
You can already review and remove flags easily from /flag/, and that UI also shows you, e.g, which tasks have been closed.
T10084 is more reasonable, and largely accomplishes the same thing anyway. Provided we implement that, after you submit changes, the flag will be more prominent when the page reloads and you can remove it with the normal "Remove Flag" action.
Nuance (T8783 / T5815) may eventually serve this use case as well, by letting you save objects to a queue instead, which sounds like it might be a closer mapping to your needs.