A11y is not very accessible which is funny. It isn't even a long word, they could just spell it out for you to make the writing more accessible.
If you are into making software for something more than just the naive "one market" your first introduction would be internationalization which should be the most widespread. When you make things for government in the EU/US you will most probably have laws demanding accessibility features. The problem isn't the numeronym but that so few people care about accessibility.