What's an overlay for web accessibility?

Automated solutions that claim to both detect web accessibility issues and fix them in compliance with the WCAG. Implemented via JavaScript code snippet or on behalf of the host site and work to adress a11y issues using heuristic engines that 'fix' issues directly on the website interface.
Most are displayed as a toolbar or widget a11y options that allow users to adjust the experience. These vendors promise to provide one-click compliance with a11y overlay tools.

My vision in a nutshell

I don't recommend using permanent overlay plugins.
We view these kinds of products as actively harmfull, and a step backwards for digital accessibility efforts.
Also, Over 250 lawsuits in the USA were filed in 2020 against companies using widgets or overlays as an accessibility solution for their websites.