In another lifetime (early RedHat days, I think. Maybe Minix, probably not) I could customise the LogIn screen's welcome message using a variety of escape codes & EnvVars. (Just to clarify: I'm talking about a msg displayed statically on the LogIn screen, not the motd or welcome msg displayed once a user logged in!)
Things like
%n for host name or %t for system time, etc., as well as a similar set for formatting. CR's, NL's etc. It may even have been more like $HOSTNAME or $TIME but I don't rightly remember... too long ago. 
So although it couldn't display a pretty backdrop or a real-time clock it could be set up to display more or less what we use Conky to do now, except on the LogIn screen. (From vague memory, rather than a text string I was redirecting from a file, but that's beside the point.)
Anyways... is it still possible to do this sort of jiggery-pokery on the LogIn and/or Lock Screen? If so, could someone point me at a list of what esc codes are available?
Ummm... come to that, where does Menu>Administration>LogIn Window (cinnamon) actually save the welcome text? It's not where I would expect,
/etc/issue, nor in the /etc/lightdm dir tree.
I don't have any option to set a message in Login-Window (18.3 and 19 with default slick-greeter / lightDM combo).
Considering the differences between distros and my... outdated knowledge, I'm beginning to feel like the old mechanic who could repair most cars from an old WW2 jeep through to a modern Lambourghini but can lose hours just finding where they put the @#%#$% start button on this thing! 