Cynically speaking, it won't matter if Brave continues to support the web request API, since all the other chromium variants aren't going to be. Once the market for extensions is 99% unable to support an API, it won't matter if the remaining 1% still does; developers simply won't target it.
What's your source on Vivaldi abandoning it? Last I heard they hadn't chosen yet. If they go full Manifest V3 then I'll have to abandon my thoughts of returning to Vivaldi (on Firefox now but there are some annoyances).
> First, Google decided to push on with discontinuing APIs used by several content blockers from the extension manifest v3. At the time, we made a promise to find a solution.
>
> Keeping support for the affected extensions (as with anything that gets discontinued in Chromium) would have been hard. Google usually removes most of the code a discontinued feature depends on and refactors anything that code relies on. So after a few versions, you end up with a patch that’s tough to apply every time.
I am reaching a little but their use of past tense makes me feel like they've decided.
It is just as much Google as Vivaldi is google because both uses Chromium.