Techbase has an "Accessibility and Usability Checklists" page, last edited in 2012.

Is this still up-to-date and relevant? Should it be merged into the Human Interface Guidelines?

NB: I have also asked about this on the kde-accessibility mailing list.

I talked to @frederik a year ago and he wanted to merge/update the guidelines, IIRC. From my side: Yes, add an 'accessibility' section to the respective pages.