But there is a reason an organization might not want to maintain Manifest V2 that has nothing to do with whether they have the ability: namely, V2 has been a massive security headache: any V2 extension can totally pwn the browser. Brave and Microsoft have made changes to Chromium that they care about (i.e., not V2) and they continue to maintain those changes.

P.S. Brave has integrated ad-blocking (i.e., blocking not implemented as an extension) which some say is comparable to Ublock Origin.