As suggested by Simon above, I asked BIAB's support team for the link to the beta version of the patch which they kindly sent me.
Unfortunately the installation failed because it requires BIAB 2020 and I own BIAB 2019.
The paradox here is that I didn't upgrade to the 2020 version because it was not compatible with Big Sur but now, to be able to try out a beta patch that can potentially ensure that compatibility... I'd have to purchase the upgrade.
I'm sure it's not meant to be like that, but it definitely feels like ransomware. In order to recover my previous investment on BIAB I'd have to pay an extra fee to PGMusic with even no guarantee that it will work.
I'm not happy!!!