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They are actually flags which have been flown at events. If you don’t believe me look up images of “straight pride” you will find pics of people holding and draped in them. At Pride events however you generally don’t see those but the straight ally flag which celebrates people outside the LGBTQIA+ but who show support because Pride is an LGBTQIA+ style independence day.
Generally speaking though because cisness is usually assumed until proven otherwise cis people don’t generally see a need to run up a flag so it doesn’t see as much use. Because allyship is more an invisible thing you will see the ally flag on buttons and things.