The “#a11y is not accessible” discussion wants to get back to the early 2010s.
It’s a signifier, for some users (for example who are dyslexic) it’s easier to type, you need some explanation, like any in-group language. Even accessibility needs explanation.
It’s not a big deal and the energy is better used elsewhere.
https://www.a11yproject.com/posts/a11y-and-other-numeronyms/
(Edit: LOL mistyped accessibility in the hashtag…)