"For as long as we’re able (and assuming the cooperation of the extension authors), Brave will continue to support some privacy-relevant MV2 extensions—specifically AdGuard, NoScript, uBlock Origin, and uMatrix"
Why take the word of the developers of a Chromium-based browser, some of whom may not even be part of the project in the long run? Firefox is built on an entirely different engine and doesn't have this problem.
Because eventually there will be enough changes to the upstream Chromium codebase that the only way to keep these extensions working would be to stop following upstream, which would mean massively increased development costs.
https://brave.com/blog/brave-shields-manifest-v3/