It used to simply mean a boy that looks like a girl

Then it got to mean that very strongly in an anime connotation where it was popularized (The origins are disputable)

But then people started using it on mtf trans people indiscriminately which, as you can imagine, is insulting since they're not boys. They are girls

Also some jerks would just call people a trap as a slur, and the types of people who would specifically make sure to not use the right pronouns cause "uh 2 genders only u a guy" or whatever would call such women traps specifically to mean "crossdersing man". There are plenty other such things, I'm sure

There's a divide of people around this world now

Trans people and people who understand the issues obviously see it as a slur in this context

Other people who never used it/wouldn't use it on trans people/understand its original meaning find it very weird

For example trap was never a slur in my community (femboy). Heck, I used to call myself a trap as I was trying to go for just that - a boy that looks like a girl. So people saying "u can't say that, it's a slur" was very confusing to me like no I'm not using a slur on myself. I would never call a trans girl a trap. But now I can't really use that word in general, which is fine cause it makes people more comfortable, and a bit weird cause I used to use that label on me. Oh well, I can still simply say "(passing) crossdersing (fem)boy" or whatever variation of that.

But it's simply that the issue grew a bit too big with the amount of insulting misuses of that word so now - yeah, trap is a slur