Microsoft Will Adopt Google Chrome's Controversial Manifest V3 in Edge (theregister.com)

The title of the OP (namely, "Microsoft Will Adopt Google Chrome's Controversial Manifest V3 in Edge") omits the crux of the issue. In particular, just because Microsoft plans to add V3 does not automatically imply that it plans to remove the API that makes Ublock Origin better than any adblocker available on, e.g., Safari. The crux of the issue is that Microsoft does in fact plan to remove the API that makes Ublock Origin better:

"We're replacing Web Request API with Declarative Net Request API, but will continue to keep Web Request API's observational capabilities" [0] -- which clearly implies that they plan to remove Web Request API's blocking capabilities.

[0] https://docs.microsoft.com/en-us/microsoft-edge/extensions-c...

I wonder if Vivaldi is considering doing the same thing?

I assume it’s more like “Microsoft will try to keep their Chromium-derivative’s codebase from diverging too much from upstream so they don’t have to maintain it after Chrome removes the old one”, and I also totally understand that desire.