Is this the same on all Linux variants?

What do you consider data files?

Is this the same on all Linux variants?

It doesn't matter if this freedesktop specification is not supported on a user's system. It's more or less just a suggestion on how to organize files in the user's home directory. You can and should always use fallback paths if the env vars are not set, which is even the default on most distros. An algorithm like this will work as a migration path for all of the env vars.

# just an example regarding the config files

# use "${HOME}/.config" as fallback path if "${XDG_CONFIG_HOME}" is empty
CONFIG_DIR="${XDG_CONFIG_HOME:-${HOME}/.config}"

if [ -d "${CONFIG_DIR}/chatty" ]; then
  # if "${CONFIG_DIR}/chatty" exists, use it
else if [ -d "${HOME}/.chatty" ]; then
  # if "${HOME}/.chatty" exists, read files from there, but write new files into "${CONFIG_DIR}/chatty"
else
  # create "${CONFIG_DIR}/chatty"
fi

What do you consider data files?

The definition is a bit blurry here and there are already suggestions to further split it up with the introduction of another "state" env var, but for now you can treat data files as files which are neither config files nor explicit cache files, like log files for example. Data files are user-specific files similar to the system's /usr/share directory.

See the table at the bottom of the Debian wiki:
https://wiki.debian.org/XDGBaseDirectorySpecification

Or see the Arch Linux wiki which also includes a list of applications which have migrated in the past:
https://wiki.archlinux.org/index.php/XDG_Base_Directory