~/.cache and ~/.config and ~/.local/share and ~/.local/state and ~/.local/bin
I used to get annoyed by non-compliance to XDG. Now I wonder if I'd actually prefer apps to reverse the hierarchy (eg, ~/.apps/nvim/{cache,config,state}).
Make it clear what needs to be backed up, what is ephemeral, and so on. Just put everything in ~/.cache. Chromium in particular is bad at this and has many types of cache.
This is a huge part of why I like docker-compose and docker in general, I can put everything I need to backup in a set of volume maps next to each other.
Is the spec perfect? No, of course not. But is it thoughtful, and does it address genuine needs? Yes, certainly.
a) store caches & libdata on different disk
b) consistently 'reset' cached data for kiosk style logins
c) make config read-only, or reset to a known good state
d) Roaming profiles where the cache is excluded from sync across machines
Most computers + home directories are 'personal' where this largly doesn't matter, but there are often sound operational reasons for this seperation in cases where you are responsible for a fleet of computers. I too perfer the 'everything related to this app in one dir' approach. Crazy idea: for apps adhering to XDG, you could point all these vars at a directory under a FUSE-style mount, which then remaps the storage any way you'd like. :)