It was Red Hat specific when it was introduced in 2004, was never supported by mount(8) upstream, and was removed entirely in 2008.
It’s confusing for GLib to keep references to it around.
Thanks to Karel Zak for digging up the history of it: !2298 (comment 1294519)
And thanks to Xidorn Quan for looking into it in the first place (see !2298 (closed)).
Signed-off-by: Philip Withnall email@example.com