Many Chromium-based browsers will keep Manifest v2 support for a while. But eventually the upstream Chromium codebase will diverge enough that it becomes too much work to keep it and they will be forced to drop it as well.
The manifest situation simply doesn't apply to Brave in relation to adblockers specifically. That is, Brave will function like uBlock without having to install uBlock as an extension - that's kinda the whole point of Brave (blocking ads / making them opt in only). That said, it is true extensions one may use that are affected by the manifest version change may be affected in Brave.
