Looks like the above method no longer works, certainly not in Big Sur 11.4, possibly before.
@Slice reminded us that Gatekeeper can be used as an alternative. Many people often disable Gatekeeper by default through Terminal command:
sudo spctl --master-disable
Kexts can be cached from /L/E in Big Sur (and Monterey) by re-enabling Gatekeeper, copying the kexts to /L/E through Terminal via sudo commands and authorizing the kexts from Security & Privacy PrefPane. Upon such authorisation, Gatekeeper will rebuild the cache and restart macOS with the newly cached kexts. Gatekeeper can then be disabled again afterwards.
sudo spctl --master-enable
sudo cp -Rf <path>/xxxx.kext /L*/E*/
GateKeeper can be disabled again afterwards.