I had a similar situation: no shortcuts containing CTRL SHIFT were working after upgrading to 20.04 (re-open closed tabs (CTRL SHIFT T), move windows between workspaces (CTRL SHIFT ALT arrow), redo (CTRL SHIFT Z), open incognito window in Chrome (CTRL SHIFT N), etc.). Other keyboard shortcuts were OK.
I have checked on the keyboard layout. Pressing CTRL showed, Pressing SHIFT showed. But when pressing both, nothing: keyboard layout not showing CTRL SHIFT .
It was like something was stealing my CTRL SHIFTs!
I tried everything, the accepted answer and these, too: https://silvae86.github.io/2020/05/12/no-keyboard-shortcuts-ubuntu-20-04/. No help.
Then I have found this: CTRL SHIFT assigned to keyboard layout change. In the Tweak Tool under Keyboard and Mouse/Additional Layout options/Switching to another layout had CTRL SHIFT checked! I have unchecked it and now I can use all my favorite shortcuts (only now I've realized that there are so many).
How to install the Tweak Tool is described here: https://linuxconfig.org/how-to-install-tweak-tool-on-ubuntu-20-04-lts-focal-fossa-linux.