Mac font suitcase files are still recognized and supported on macOS Big Sur (11.x), but not over local networks anymore. The apparent reason for this is Apple reducing support for AFP (Apple Filing Protocol) network protocol. SMB has been the default at least since Catalina, but you could enable AFP prior to Big Sur, at least for HFS volumes. That option has been dropped.
Volumes formatted in APFS (which supersedes the ancient HFS) have never been able to be shared over AFP, so this appears to be part of Apple's ongoing effort to phase out HFS.
If you have a pre-Big Sur Mac on a network, it can still serve HFS volumes over the network and they are still recognized and mountable by Macs running Big Sur.
All this is preface to say that you need to be careful if you still have old font suitcase files that you want to maintain or continue to use.
Font suitcases date back to the pre-OS X classic Mac OS of the eighties and nineties. The classic Mac OS file system supported files with two "forks": a data fork and a resource fork. The data fork was were user data was normally stored, for example in document files. The resource fork held metadata (such as file type and creator codes) and also system resources, such as fonts. Font suitcases were basically files that contained nothing but font data in the resource fork (and nothing at all in the data fork).
"Foreign" operating systems, such as Windows, UNIX, and Linux, have no support for this two-fork file structure and can only "see" the data fork. So if you copy such a file to such an OS, only the data fork survives and the resource fork is lost. Since font suitcases contain all the data in the resource fork, you end up with an empty file (zero k).
Mac OS X/macOS has always included some support for this non-standard file structure. You can still install and use ancient font suitcases on modern Macs for either Type 1 or TrueType fonts. (Not old bitmap font formats, however.) But SMB, which comes from the Windows world, does not appear to recognize resource forks even on macOS, so it's the same result as trying to move a font file to Windows or over the internet (which uses UNIX file protocols).
Consequently, if you want to preserve old Mac font suitcases when transferring over a local network under Big Sur, you will need to store them in a .zip archive, as if you were emailing them or moving them through a non-Mac operating system. Physical transfer, via a Mac-formatted removable media, still works fine.
FWIW, this is also the case with iCloud, although I don't know what file protocol is used for that. iCloud, for whatever reason, does not support files with resource forks.
I'm a little disappointed by this, but at least Apple hasn't totally dropped support for font suitcases.