Keeping the power button pressed for about 15 seconds will force a reboot.
Keeping Volume + pressed while starting/rebooting the phone will boot the phone into recovery mode. A factory reset is an option there, but you would have to actively select and confirm it.
Keeping Volume - pressed while starting/rebooting the phone will boot the phone into Fastboot Mode. You would need to connect the phone to a computer and know what exactly to do then to delete user data with fastboot.
When something went wrong, I’ve seen the phone boot into a prompt which offered a factory reset as an option, but you would still need to actively choose or confirm to do that. This was also mentioned in the forum announcement about this update …
I don’t think a key combo would do a factory reset, this would be much too dangerous, there would always be somebody accidentally resetting their phones.
If a data loss without user interaction really happened I would rather think the update somehow did it, which obviously would be one of the worst issues imaginable, but I don’t have the retracted file to try to check this, and the announcement only said …