I'm using Ubuntu 18.04.5. Following the manual from https://help.ubuntu.com/stable/ubuntu-help/shell-windows-tiled.html neither of two methods of horizontal window tiling works. Dragging window's title bar works only for maximizing it, not tiling.
I don't recall installing any applications, which could affect this behavior. I have Gnome Tweaks installed, but I don't see an option for tiling there. Any clues how to restore this functionality? Is it broken in Ubuntu 18.04?
Check "Gnome Tweaks" again, on the Windows tab. There is an option "Edge Tiling" that needs to be on to enable left/right tiling by dragging to the edge of the screen. Alternatively, enable the setting with the command
Then check that your keybindings for tiling are correctly assigned. You can reset to the default with the following two terminal commands: