I got used to splitting my application windows on monitor into 4 sides top-left, top-right, bottom-left, bottom-right with key combinations like Ctrl+Alt+Key3 and so on.
It no longer works with Ubuntu 18.04. Is there any alternative for that with Ubuntu 18.04?
Ubuntu 18.04 dropped support for the Unity desktop compositor in favour of GNOME Shell. GNOME Shell is incompatible with Compiz. You need to look for a method to replicate this behaviour using GNOME Shell and its extensions or install and use an unofficial, community-supported version of Unity.
P.S.: There's a follow-up question with a suggestion for a replacement GNOME Shell extension.
Here you can find the appropriate GNOME extension: https://extensions.gnome.org/extension/39/put-windows/
Install gnome-session-flashback, or Mate DE, and then install compiz and all is as it should be.
Here's the instructions on how to install the Gnome Extension "Put Windows", which will give you the keyboard shortcuts you are looking for to replicate the window arrangements you want. You can snap windows to the top-right, top-left, bottom-right, bottom-left, top, bottom, left, right, or center. Repeated presses of the shrotcuts even can toggle different size arrangements of many of those.
Follow the instructions here: How can I snap a window in a corner with 18.04?
UNFORTUNATELY, it seems this extension does not enable the drag-to-corner feature to make it snap to the corner. Rather, you are left only with Gnome's default drag window to the left to snap left, and drag window to the right to snap right. Dragging to the corners will NOT make them snap to the corner. You must use the keyboard shortcut for that. IF ANYONE FINDS A WORK-AROUND TO MAKE DRAG-AND-SNAP-TO-CORNER ACTIONS POSSIBLE, PLEASE LET ME KNOW.