There is a changed version that includes drop-in config files, masking, and loading defaults from /usr/ instead of requiring full config files to live under /etc:
https://uapi-group.org/specifications/specs/base_directory_s...
«basedir: support separate (vendor/local) trees and masking for config files» https://gitlab.freedesktop.org/xdg/xdg-specs/-/merge_request...
«basedir: support drop-ins for configuration paths» https://gitlab.freedesktop.org/xdg/xdg-specs/-/merge_request...