I'm pretty sure the XDG base directory spec was not specifically a gnome thing. It came from freedesktop.org[1] if I remember correctly, and in fact gnome was not compliant to begin with[2]. That gnome wiki page references that the recommendation (which turned into the spec) came from freedesktop and documents the gnome "initiative" to become compliant. Freedesktop was focussed on compatibility between desktop environments and systems, so isn't just a linux thing.

[1] https://specifications.freedesktop.org/basedir-spec/latest/ [2] https://wiki.gnome.org/action/show/Initiatives/GnomeGoals/XD...