SUMMARY
It is possible to create a bond over WiFi, but it can't be done in the plasma-nm connection editor. Once you create a bond and you can add/edit the lower interfaces, the option to add a Wireless interface is not there. 

It is there for Bridges, but WiFi can't be added to bridges unless you enable WiFi 4Addr or run the WiFi in AP mode (with hostapd).

STEPS TO REPRODUCE
1. Open the connection editor
2. Add a new Bond interface (or edit one you already have)
3. Click the "+ Add" button

OBSERVED RESULT
Options available are "Ethernet" and "Infiniband"

EXPECTED RESULT
Options should include WiFi

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Linux 5.7.5, openSUSE Tumbleweed 20200701
KDE Plasma Version: 5.19.2
KDE Frameworks Version: 5.71.0
Qt Version: 5.15.0

ADDITIONAL INFORMATION
See Bug 423940 for a working configuration of this set up.