The issue with enabling this is that allowing snaps to be uploaded with compression formats that are not universally supported like XZ and LZO introduces the possibility of folks uploading snaps that cannot be used on platforms that we, Canonical and the snapd team, specifically support.
You have mentioned a custom snap store, if you have a brand store I would suggest reaching out to your sales rep folks about the possibility of enabling this for your brand store, it may be something we can do for specific brand stores until we have a more general solution in place which allows us to provide a better user experience / story for users on distros which may not support new compression formats like zstd, etc. See my explanation here for more details: https://ubuntu.com/blog/why-lzo-was-chosen-as-the-new-compression-method
Regarding the patch to snapd, I’m sure it’s extremely simple, but we can’t accept it until we have a better story for users on old distributions that don’t support the new formats.
We could easily probably add support for GZIP and “none” compression methods since those should be easily supported everywhere but indeed the more interesting thing is ZSTD which brings the best of both worlds for both size and speed.