I mean I get it, but accessibility is one of those things that can be very painful to add after the fact. If this project is meant to be used in actual applications, they’ve pushed a very tedious task unto outside contributors.

This is a fine and good statement for many things, but a11y is more of a fundamental architectural & design consideration for projects of this variety. In my experience, a lack of a11y consideration overall is more of an indicator of the overall suitability of the product design for your needs than simply a "missing feature you can add".

Open Source software is great insofar as you can contribute to it in many ways, but applying this logic to all situations is really dismissive. There are many aspects of software projects that are largely or solely dictated by the creators/maintainers of that software, that may be impossible for idle contributors to significantly affect (and even forking can be a gargantuan/unviable option in some cases).

TL;DR: Open-sourcing your software should not shield you from constructive criticism, NOR should it invalidate the advice of informed people recommending against using your software.

The fact that "anyone can add a feature" doesn't excuse the lack of a feature existing _right now_, especially considering (as other commenters point out) accessibility is very difficult to retrofit. Not to mention, accessibility should be a top priority for any user-facing software, and the fact that it was utterly neglected suggests there may be other basic flaws.