Last chance for boarding varies from one airline to anothe also. I think Delta gives away seats (not just the assignments) to standbys ten minutes before "departure time". Also Delta expects (or used to expect) you to have checked baggage in by 30 minutes before departure time, otherwise you may be bumped then and there with no compensation.
I think United "closes the door" ten minutes before departure.
However on my last WDW trip (Nov. 01) Delta held some flights for people stuck in security lines, my flight out of BOS was held 20 minutes.
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