I'm surprised YouTube doesn't just block the Brave browser entirely. The entire business model of Brave is "siphon off some of the ad revenue from websites".
Do you mean Microsoft or something else I might be missing? In Microsoft's case, they were punished for their OS monopoly being used to create a browser monopoly by making a default choice for the user, not for abusing its browser monopoly.
Blocking Brave for violating their usage agreements seems very much in another league, even though it might sound like they are building a browser monopoly via their video streaming monopoly. They are not in a position where they take the choice of defaults away from the user, and as long as Firefox, Safari and Edge have some market share, the user still has a choice of compliant browsers. Blocking Brave might be more akin to enforcing their ToS for Youtube via their Play store by blocking 3rd party apps with download function (which is prohibited in Youtube ToS).
