This isn't true. I'm not a Brave user (I use FF), but Brave have said that
1/ they have adblocking built-in [0]
2/ they will continue to support Manifest V2 indefinitely, esp. so that uBlock Origin continues to work [1]
That doesn't mean uBlock will continue to exist... If it can't be installed on Chrome it's possible the sole dev will lose motivation (although he has already made a V3 version of the extension).