If I'm being honest, I think this is a fundamentally misguided approach.
Itch.io doesn't create the tags - people type in the tags and choose ones that they wish to have. I've seen tags that exist for one game only - I've seen tags that are extremely misspelled. Etc etc etc. Itch.io didn't designate 'LGBT' as the tag. People did.
Right or wrong, it's highly unlikely that Itch.io would go back and change an entire tag to be another thing, and not allow people to tag their works 'LGBT'. For better or for worse, that is what people chose to tag their works, and forcing people to use a different tag seems like a very poor idea overall.
If you want the 'LGBTQIA' tag to be larger, that's something to take up with the community. If you (general you) create games, tag them with that. Perhaps create a game jam where half the point is to put your works in that tag? That would not only create more games, but also draw attention to it. (Even if people couldn't create games for that specific game jam, if they had before, perhaps they'd see that and change whatever tag they were using.) That would be my best suggestion, as well as simply bringing it up. For example, the Rainbow Game Jam has a discord (as do most of the game jams, when they're active). Join it and bring it up there, as an example?
If you want the 'LGBTQIA' tag to be more commonly used, I truly think the community is the way to start. From how itch.io genuinely functions as a website, I think it's highly unlikely they'd change it - as, again, it wasn't a tag they truly created in the first place - so I think if you want it to be a more common tag, starting with the community is the better way to go.
Cheers, and good luck!