Quoting Steve Litt (slitt@???):

> I looked this up, and the only reference I saw was
> https://wiki.archlinux.org/index.php/SLiM#Environments, and that didn't
> make it sticky, it just made your .xinitrc DEFAULTSESSION the default
> if you chose something outside of the .xinitrc case statement, when in
> slim.

As a reminder, that page says that it's necessary to edit _both_
/etc/slim.conf and ~/.xinitrc.

> Stickiness isn't the ability to state a preference. It's the ability to
> keep whatever you chose last time, until you deliberately choose
> something else.

SLiM doesn't support that. The page says so right under subhead
'Configuration'.

If you want to add that feature, submit a patch.

(I don't have a setup that includes SLiM, preferring as I do something
simpler.)