I noticed that xdg.vim was added after the discussion in #19421.

It seems the approach is to have a way for users to do :ru xdg.vim and have the XDG paths set for them automatically, fair enough.

I see a couple of problems with this. First, the file says:

  " These options are not set by default because they change the behavior of
  " where files are saved. Uncomment them if you want to fully move all
  " transient/persistent files to XDG directories.

Now let's consider that $VIMRUNTIME isn't always writable by the user, they might not have root access. Furthermore, making changes to files in /usr isn't good practice because an update can overwrite them.

Users would have to make the change in their ~/.config/vim/vimrc anyway. With this in mind, the xdg.vim feels redundant.

Correct me if I'm wrong, but I think a better approach would have been to provide instructions for setting such paths rather than providing xdg.vim.