I’m not sure about the changes since 18.04, as I only just started using Lubuntu myself yesterday. In 20.04, you can add hot keys to do this to your ~/.config/openbox/lxqt-rc.xml file. Presumably this is what 18.04 included by default.
Using the [Super] key seems to be a bit tricky, because it conflicts with the hot key for opening the LXQt main menu. Any other unused key combinations are usable, though.
In the example below, I’ve used [Control]+[Shift]+[arrows].
In the <keyboard>
section in your ~/.config/openbox/lxqt-rc.xml file, add:
<keybind key="C-S-Left">
<action name="UnmaximizeFull"/>
<action name="MoveResizeTo">
<width>50%</width>
<height>100%</height>
<x>0%</x>
<y>0%</y>
</action>
</keybind>
<keybind key="C-S-Right">
<action name="UnmaximizeFull"/>
<action name="MoveResizeTo">
<width>50%</width>
<height>100%</height>
<x>50%</x>
<y>0%</y>
</action>
</keybind>
<keybind key="C-S-Down">
<action name="UnmaximizeFull"/>
<action name="MoveResizeTo">
<width>100%</width>
<height>50%</height>
<x>0%</x>
<y>50%</y>
</action>
</keybind>
<keybind key="C-S-Up">
<action name="UnmaximizeFull"/>
<action name="MoveResizeTo">
<width>100%</width>
<height>50%</height>
<x>0%</x>
<y>0%</y>
</action>
</keybind>
Save the file, then open a terminal and enter the command openbox --reconfigure to make the changes take effect.