Newer versions of sddm display manager do not add entries from /usr/share/xsessions/ files with "NoDisplay=true".
https://github.com/sddm/sddm/commit/cf4c3caf058ea81d0e61c2087493623faac56a3f
After updating to new sddm the entry for xmonad session is missing.
Closes: https://github.com/gentoo/gentoo/pull/9403