Inside WCAG 2.2 REC there are other wcag 2.1 understanding references:

Accessibility supported definition

note 1:
<a href="https://www.w3.org/WAI/WCAG21/Understanding/conformance#support-level">Level of Assistive Technology Support Needed for "Accessibility Support"</a>

note 5:
<p>.... (See <a href="https://www.w3.org/WAI/WCAG21/Understanding/conformance#documented-lists">Understanding Accessibility-Supported Web Technology Uses</a>.) </p>

Conforming alternate version (last paragraph)
<p>See <a href="https://www.w3.org/WAI/WCAG21/Understanding/conformance#conforming-alt-versions">Understanding Conforming Alternate Versions</a></p>

Text alternative definition (note already in #3486 )
<p >Refer to <a href="https://www.w3.org/WAI/WCAG21/Understanding/conformance#text-alternatives">Understanding Text Alternatives</a> for more information.</p>