This is a really well written article. It's unfortunate that this person has to deal with all these issues.
I suspect the reason that both MATE and the Debian installer have good accessibility support is because their codebases are quite old. In general, it seems like older software is more likely to have better accessibility support than newer software.
Accessibility should be something that's built into software from the very beginning, but I totally understand that not all developers have time for it or properly understand it. It's unfortunate.
Edit: I forgot to mention that accessibility is going to be mandated for some types of sites and apps in the EU thanks to the European Accessibility Act (EAA) coming into effect later this year, which should help somewhat. Won't really help with Linux itself though.