SUMMARY
There is currently no way to effectively navigate through the first level of sidebar view of System Settings using Tab, Shift+Tab, Ctrl+Tab, Enter, or direction keys.
At most, pressing Tab once will remove focus from search bar, and Tab a few more times will cycle through the Frequently Used section with no possibility of input whatsoever aside from cycling with Tab and Shift+Tab.
Ctrl+f does not bring focus back to the search field. This occurs on tree view and icon view as well.

There's no indication whatsoever that Ctrl+m will bring out the Configure window under the hamburger menu, even when pressing Alt. This occurs in all view modes.

This is confusing since the second and third levels of System Settings allow for keyboard navigation.
This leads to two situations:
*A keyboard-centric user, confronted with the first level of System Settings, sees the inability to navigate through it without the mouse, and thus thinks the rest of System Settings does not allow for keyboard navigation; or
*A keyboard-centric user that accidentally finds out the second and third levels of System Settings are keyboard-navigable might expect the first level to be as well, and be frustrated with the inability to do so.

It's worth mentioning that both tree view and icon view allow for basic keyboard navigation.

SOFTWARE/OS VERSIONS
Operating System: Kubuntu 19.04
KDE Plasma Version: 5.15.4
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.2
Kernel Version: 5.0.0-13-generic