I apologise for not following up with my bug reports. Here is the latest 
spec and a thread from 2014 that explains defaults.list appears to be 
long since unused, that debian uses it only for historical reasons that 
are perhaps a decade old.

It was mentioned in this 6 year old thread that GNOME was using 
defaults.list. Is there any reason to think that it does now and that we 
shouldn't just set it to mimeapps.list? I haven't gone into GNOME 
sources to figure out what it ultimately does.

-- https://specifications.freedesktop.org/mime-apps-spec/latest/ar01s02.html

As you can see, there is no defaults.list specified at all.

-- https://lists.freedesktop.org/archives/xdg/2014-February/013183.html


As for should XDG_CURRENT_DESKTOP, having a brief look at the spec, this 
is merely a way to make desktop items appear in or not appear in the 
menus of a certain desktop environment. In a .desktop file you can have 
smoething like |OnlyShowIn|=GNOME or |NotShowIn=XFCE. If the respective 
desktop environment you are using follows the standard it will check 
these to decide if it should show or hide the entry. I believe it is 
unrelated to the defaults.list vs mimeapps.list problem.|

||-- 
https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html