Queer has absolutely been used as a slur that contributed to the repression of several people I know personally, for one.
For two, "it's unwieldy" isn't really a good enough reason to shortcut on representation - the reason LGBT caught on is because it started as LGBT, and the QIA+ was added as it became apparent there were people who should be together under the umbrella but weren't represented. It's also simply not unwieldy in the context of a game tag - the full tag will come up when you type LGBT while adding a tag, and it will be a suggested tag if you search LGBT. I don't know how the itch search actually works but I wouldn't be surprised if just searching for LGBT also matched the LGBTQIA+ tag. So it really isn't unwieldy in this context, and the more it's used in common situations like this, the less "unwieldy" it'll be in normal conversation - it'll just be "correct"