I feel like trap has as of late been slowly replaced with "tomgirl" in many circles I've browsed. It often just means someone who's secretly a boy but doesn't disclose it and is mistaken for a girl. Sometime it's done with intent, I find that more often it's a "oh you never asked if I was a boy"

This could be a very relative POV based on what stuff I browse but I often found that trans wasn't really a thing in hentai til recently. You had futa, a girl who "was just born this way" or had a curse or whatever, or trap, a crossdresser who's "super-gay" or unintentionally feminine. No in-between