Apple's apps bypass firewalls like LittleSnitch and LuLu on macOS Big Sur

I wonder if it would make sense for Little Snitch to continue supporting their kext-based solution in parallel to the new one, possibly only for users who are willing to disable SIP.

You might argue that disabling SIP for a security product defeats the point, but I'm not sure if that's necessarily true. SIP effectively delegates trust away from the user and towards Apple, which is fine as a default—but the calculus may be different for experienced users, like the ones who use Little Snitch.

Eventually I don't think little snitch will even have apis to access stuff like that in the kernel as a kext as macos updates continue on.

Kexts are used by Apple internally, so I'd be shocked if they were removed from the OS completely. Third party kexts may be deprecated, but as long as SIP can be disabled it will always be possible to load your own.