I cannot create global shortcuts with the tab key any more. I used to set the workspace grid to the combination Meta+Tab but in Plasma 5.19.4 it does not work because while pressing the tab button switches the focus of a button and does not get recognized as a shortcut key press.
This definitely did work in an older version because that's a shortcut I set in all my installations.

STEPS TO REPRODUCE
1. Try to edit a (global) shortcut in Plasma settings.
2. Try to make a shortcut like Meta+Tab or Alt+Tab or something like this
3. Observe that it will no longer work.

OBSERVED RESULT
Tab should be recognized.

EXPECTED RESULT
Tab switches focus of a button.

SOFTWARE/OS VERSIONS

Operating System: openSUSE Tumbleweed 20200801
KDE Plasma Version: 5.19.4
KDE Frameworks Version: 5.72.0
Qt Version: 5.15.0
Kernel Version: 5.7.11-1-default
OS Type: 64-bit
Processors: 8 × Intel® Core™ i5-8250U CPU @ 1.60GHz
Memory: 7.6 GiB of RAM
Graphics Processor: Mesa DRI Intel® UHD Graphics 620

ADDITIONAL INFORMATION

However a workaround can be used to use the tab key anyway: export your current hotkeys and edit the line with Kate manually. Then import the hotkeys and the shortcut will work.