The OP in this thread was heading in the almost right direction. It seems that making the Compose Key persistent on Lubuntu requires editing:
Blockquote
/etc/X11/xorg.conf.d/10-keyboard.conf
The file itself warns against being edited, saying in bright red letters: “# Read and parsed by systemd-localed. It’s probably wise not to edit this file manually too freely.”
Gentoo wiki where I initially found how to do it also says:
Blockquote
For sake of sanity, it is advised to use of a proper desktop environment with its own keyboard layout management.
Unfortunately, I was unsuccessful finding the GUI option in Lubuntu (did find it in Ubuntu though) or in its manual. I assume that it does exist and I just can’t find it. Could someone please point me to a place where this can be done without manually editing system files in the future?
In the meantime, the Arch Wiki provided me with an easy-to-follow example and my 10-keyboard.conf file now looks like this:
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Section “InputClass”
Identifier “system-keyboard”
MatchIsKeyboard “on”
Option “XkbLayout” “us”
Option “XkbOptions” “compose:ralt”
EndSection
And it works! My Right Alt now serves as the Compose Key and the configuration persists after a reboot.