I think when you say “all the linux versions” it is pretty vague and obviously impossible to support or estimate. Our goal is to support snapd in all the places that matters to users and where it is technically possible to run snaps.
Snapd itself depends on the Linux kernel (so very old / android kernels are not an easy support target), systemd (which has similar requirements) and a few userspace utilities.
We are constantly working on improving support of the major distributions, by extending our testing system and working on packaging both upstream and downstream. Anyone is welcome to join this effort so if something specific (a given distribution or environment) is important to you then please, do join us in this effort.