Glad that wifi calling stayed on for you. For me (S20, not O2 firmware but with an O2 pay monthly sim) it disappeared again after the Jan update.
I still have to power off, remove sim, power on then reinsert sim after each restart.
In my case I don't need O2 firmware to enable wifi calling and VOLTE and it worked fine for six months until the December update. I had hoped that the January update would fix it but sadly not.
I too do not have O2 firmware and did not have a problem until recently, when I had to use the insert sim once booted trick. Also O2 pay monthly sim. Odd we have differing results (although different model variant phone noted).