There is still support for CSM in the open source https://github.com/tianocore/tianocore.github.io/wiki/Compat... and even nice projects like https://github.com/coreboot/seabiosto fabricate the CSM16 binary, but many vendors have stopped validating this path, including production of legacy BIOS option roms for adapters (net, gfx, etc) https://www.phoronix.com/news/Intel-Legacy-BIOS-EOL-2020. I still believe CSP's maintain some of this support in their hypervisors' guest firmware for legacy OS binaries/ISO boot support? Also since Windows requires UEFI Secure boot enabled by default and CSM has to be disabled for the UEFI secure boot path, this is another reason legacy BIOS boot isn't exercised so much these days.
We could have added legacy oroms hashes to UEFI Secure boot implementations https://patents.google.com/patent/US8694761B2/en, too, but again folks pushed back in their zeal to remove legacy BIOS overall.
We didn't add the CSM spec https://www.intel.com/content/dam/www/public/us/en/documents... to the PI https://uefi.org/specs/PI/1.8A/ since folks were hoping UEFI would remove the need for CSM. I still remember being challenged in the early 2000's by a long-time BIOS mgr at Intel "Is removing legacy a good idea with EFI? You know, we're really at legacy."
