I don't think the new manifest rules are because of any actual restriction or the engine, maybe I'm wrong but it was my impression that the change is mostly just Google wanting to control what users can do with their browsers even more (and always in ways that make them profit of course). So they should be able to keep V2 support if they're willing to be on the hook for keeping it maintained themselves.

And if the removal of manifest v2 allows google to abandon/remove/refactor code exposed through it (and google could even be inclined to do so deliberately)? At some point Brave would end up having to maintain its own fork, which they probably don't have the resources to do.

Brave will keep Manifest V2 compatibility as long as they can, specifically focusing on "AdGuard AdBlocker, NoScript, uBlock Origin, and uMatrix". https://brave.com/blog/brave-shields-manifest-v3/

I guess I'm curious what "as long as they can" actually means. If it means they can't pull upstream Blink without losing v2 support, that's bad.

I think Firefox is the only viable solution to continue using UBO at this point.

I seem to recall they said as long as it was technically AND economically feasible. It depends on how much google attempts to spread the cancer of mv3 throughout their code base. Eventually brave won’t have the manpower to hack v2 in is the likely result without going bankrupt if google really wants to go down that path.