The analogy I like to use is they (Chromium-based browsers) are in another lane on the same road whereas a complete fork diverges to their own road. The former is still beholden to whatever direction the Chrome road goes.
I don't have the link handy since I'm on mobile, but they've stated in a GitHub issue that they're not going to maintain v2 support, just activate the extended enterprise support to give users more runway. This just seems to be a mock-up of a toggle for that extended enterprise support and links to the extensions since they won't be downloadable from the consumer Chrome extension store.
I will admit that they haven't been clear (perhaps intentionally), but I have yet to see an announcement that explicitly states that they will independently maintain v2 support.
With that said, Brave's built-in ad blocker should not be affected by the change.