As a more general question, is there a resource available that shows which memory protection schemes are available on various OSes? It'd be nice to see what the state of the art is on Linux, macOS, the BSDs, etc. in one place.

Different. These are enhanced mitigations to detect when a program is calling into the kernel from a weird state that should never happen in a well-formed program. It basically prevents certain types of exploits from successfully calling into the kernel.

*the new mitigation just makes user-mode memory completely immutable. You can never "upgrade" a page's permissions or change its contents after it's been marked as immutable.

Seems like the second blurb is similar to the Page Protection Layer+Pointer Authentication Codes in XNU.