There's a few solutions:
LittleSnitch 4 can continue to work (with the kext) on Big Sur following this: [1]
LittleSnitch 5 can block all protected MacOS processes by following this: [2]
Murus can use PF and block IPs for Apple services: [3]. This isn't per process, and is really just a UI for the built-in PF process.
If you'd like to block the notarization check, you can block trustd (/usr/libexec/trustd) access to ocsp.apple.com (on both system and user process ownership in LittleSnitch).
Hope this helps. It's really not as bad as you think, there's a few solutions depending how thoroughly you want to block things.
[1] - https://www.obdev.at/support/littlesnitch/245913651253917
[2] - https://tinyapps.org/blog/202010210700_whose_computer_is_it....