At least on Ubuntu 20.04 using GNOME 3.36.8, regardless of the VCL used, the shortcut Ctrl + Alt + left or right does not resize the cells of a table.

Documentation:

https://help.libreoffice.org/7.4/en-US/text/swriter/guide/table_sizing.html

Versions:

Version: 6.3.6.2
Build ID: 2196df99b074d8a661f4036fca8fa0cbfa33a497
CPU threads: 8; OS: Linux 5.15; UI render: default; VCL: gtk3; 
Locale: en-AU (en_AU.UTF-8); UI-Language: en-US
Calc: threaded

Version: 7.4.3.2 / LibreOffice Community
Build ID: 1048a8393ae2eeec98dff31b5c133c5f1d08b890
CPU threads: 8; OS: Linux 5.15; UI render: default; VCL: gtk3
Locale: de-DE (en_AU.UTF-8); UI: en-US
Calc: threaded

Even though only the Ctrl + Alt + Up or Down are used for switching workspaces in this GNOME version, I am wondering if the issue is linked to GNOME workspaces. Maybe GNOME-specific.

Side note: nothing is linked to it in Tools > Customize > Keyboard (the shortcuts for resizing columns and rows neither, but they still work). Might just mean they are not configurable.