LGBT and other variants, yeah i agree but with one exception: LGB is often used explicitly to exclude trans people and postdates LGBT in use. It's one thing to use it when there's a reason to exclude straight trans people from a statement (eg LGB people may struggle early on because they don't have a relationship script to follow in same gender relationships). But yeah, I use LGBT with the understanding that other queer identities exist and are to be included as warranted.

The + came pretty early, and the alphabet soup largely is pushed by specific groups who worry they're underrepresented or underacknowledged. This began with asexuals who were kinda late to organizing with the rest of us and felt the need to really push for visibility. The Q was basically meant to be the same as the +. The I is related to the debate as to whether or not intersex people are inherently part of our community (my stance is that it's not my place to tell any intersex person whether or not they are, though idk what my cousin would get out of it as a straight cis woman who's chromosomally sterile). By the time QIA was in the mix some people just kept going and a lot of us pulled back.

It should also be noted that LGBT was an acknowledgement of the fact that each letter had developed a separate community and we'd been working together and we had an overarching community. Over the years since a lot of that has blended, but early on a lot of organization was separated into those 4 boxes, in a way that just isn't as common anymore.