Yes, if someone take convoy and run it after the time of event is done, he still will be visible on that list and still can be plundered.

I think 21:00 is more like a deadline to select ss convoy...Plunder will be open 24:00 hours ..but convoys have a time of x minutes before which you need to reach destination..so you can plunder for convoy deadline + x minutes

if you select an ss convoy at 20:59 you have 30 minutes to deliver it to destination, so up to 21:29 is still possible to plunder somebody.