Brave said they will keep the v2 capability in their Chromium fork "as long as it's possible" (paraphrasing, I can't find the exact tweet to quote).
Specifically, "we have decided to implement DNR and continue maintaining support for blocking webRequest. Our initial goal for implementing DNR is to provide compatibility with Chrome so developers do not have to support multiple code bases if they do not want to. With both APIs supported in Firefox, developers can choose the approach that works best for them and their users."
https://blog.mozilla.org/addons/2021/05/27/manifest-v3-updat...