Here’s a collection of all the A11y resources I’ve found at WordPress.org sites (and two Google docs in use for drafting Handbook pages):
Make WordPress Accessible blog
https://make.wordpress.org/accessibility/
Make WordPress Accessible> Accessibility Handbook
https://make.wordpress.org/accessibility/handbook/
(Draft: outline for A11y Handbook) Accessibility Contributor Handbook
https://docs.google.com/spreadsheets/d/1tOYzFn9vc7Ff4yGBmDajelrl7XMDUz4PR5H2lB4exI4/edit?usp=sharing
Make WordPress Accessible> Useful Tools
WordPress Accessibility> Theme Patterns
https://github.com/wpaccessibility/a11ythemepatterns
Docs Handbook> Handbooks Style and Formatting Guide> Accessibility
Codex> Accessibility
https://codex.wordpress.org/Accessibility
Theme Handbook> Accessibility
Theme Review Handbook> Accessibility
Theme Review Handbook> Accessibility> #resources
Theme Review Handbook> Accessibility> Resources
Theme Directory> accessibility-ready tag
https://wordpress.org/themes/tags/accessibility-ready/
Plugin Directory> WP Accessibility
Plugin Directory> Access Monitor
(Draft: guidelines for Plugins) WordPress A11y Code Standards
https://docs.google.com/document/d/1iySvDJ4duHYt6YlnnjnBNbU5LKAn1uRBMH8FKAfmswE/edit
Accessibility code standards for WordPress core (similar to above)