Not just "Little Snitch", all Firewalls that had their own kernel extensions. With macOS Big Sur, Apple started insisting that thry have built specific API's for firewall softwares, and every firewall app had to use only that, and their custom kernel extensions won't be allowed. But Apple's API are ofcourse designed to allow it bypass their whitelisted softwares.

More here: https://medium.com/tripmode/apple-started-hiding-the-traffic...