User management under in Lubuntu 18.04 used to be in System Tools -> Users and User groups
.
Where is this option in the new Lubuntu 20.04 LTS released today?
It seems that Users and User Groups
has been moved to Preferences -> LxQt Settings
under the new 20.04 LTS.
I would suggest looking up the Lubuntu Manual
https://manual.lubuntu.me/stable/3/3.2/3.2.15/users_and_groups.html
A section of a Chapter exists documentation that feature.
How to Launch
To launch Users and Groups from the menu Preferences ‣ LXQt settings ‣ Users and Groups. The icon for Users and Groups is a teal circle with nondescript picture of a person. Or you can run
lxqt-admin-user
Lubuntu 18.10 up, including Lubuntu 20.04 LTS use a different desktop (LXQt) compared to LXDE found in Lubuntu 18.04 LTS and before, so it wasn't moved but is a result of the different desktop now used in the now last four releases of Lubuntu.
The Lubuntu team created the manual (introduced with 18.10) for that purpose to help users with the change of desktop.