So I wen't ahead and did the upgrade just to see...
First things first: OpenWRT does warn me this is not supported.
Result?
After checking "force", and waiting for 10 minutes+, the device is still down and diag being lighted in red. I guess I'll have to go playing with TFTP again then.
So now you know. Don't try to flash 21.02.1 g302h sysupgrade firmware on your B0B0 buffalo 