I found a Vivaldi blog post on this topic from 2022: https://vivaldi.com/blog/manifest-v3-webrequest-and-ad-blockers/

Will the Vivaldi Ad Blocker be affected by the Manifest V3 changes?

I made some architectural choices early on that I believe should keep it functional, regardless of the Manifest V3 changes. Of course, there is always a possibility that the underlying Chromium architecture will change now or in the future, forcing us to do some extra work to keep this working. ​> Hopefully, a more in-depth description of the architecture and some of the facts surrounding the Manifest V3 changes should help to show why I believe that our implementation is safe for the time being.