The longdesc attribute is marked as obsolete in WCAG 2.2 Techniques H45, but Understanding Conformance page still contains a reference to it.

Sometimes, supplemental information may be available from another page for information on a page. The longdesc attribute in HTML is an example. With longdesc, a long description of a graphic might be on a separate page that the user can jump to from the page with the graphic. This makes it clear that such content is considered part of the Web page, so that requirement #2 is satisfied for the combined set of Web pages considered as a single Web page. Alternatives can also be provided on the same page. For example creating an equivalent to a user interface control.