It's good advice for any abbreviation to define or provide the expanded form the first time it's used; The A11y Project does this in the top heading on its home page, "a11y stands for accessibility." It might be helpful for the project to include that explanation on every page, even if it's just in the footer.

"a11y" is useful as a hashtag, is shorter and easier to spell. A11y is a good search term; content containing "a11y" will be about digital accessibility. Content containing "accessibility" could be about DNS issues, account authorization, or a number of other topics even before search "helpfully" expands the search to include "accessible," "access," and other terms.