https://github.com/w3c/csswg-drafts/tree/main/css-position-4 has the source for a Level 4 spec for CSS Positioned Layout, but there’s no spec published at https://drafts.csswg.org/css-position-4.
MDN has an article on the ::backdrop pseudo-element. That feature isn’t in the Level 3 spec, so MDN currently links to https://w3c.github.io/csswg-drafts/css-position-4/#backdrop, which did exist and work as expected until the 52077ef change was made. And in order to fix that, there needs to be a published https://drafts.csswg.org/css-position-4 spec that MDN can link to.