They will still be shipping 18.04lts, with security updates, for years to come. This will be increasingly outdated. I'd like to hear specific examples of where that would be a problem though, besides Wine. More specifically, I don't understand what Valve is so upset about since Valve already supplies their own run time that games are recommended to use, and I hope that would apply to their photon system.
Snaps can be built from an OS version, e.g. from 18.04 which still has 32-bit and will for 5 years (from April 2018). And the snap includes all the system libraries etc.
Fwiw, 18.04 is promised security updates until 2028 instead of the normal 2023.
Only if you're paying for the extra support through Ubuntu Advantage program.
I haven't actually used that function myself, but snapcraft supposedly supports docker as a building tool... So I'd imagine you could just use Alpine as base