With the upcoming removal of manifest v2 (which I presume Brave is not committed to continue supporting), there will no longer be a way to create userscript extensions (such as Greasemonkey, Tampermonkey, and Violentmonkey) for Chromium-based browsers. This will have a very negative impact on many users (over 10 million people use Tampermonkey, for instance) who use userscripts for a wide variety of purposes to fix/optimize websites at a user level. In short, they are extremely useful. I myself use them on all sorts of sites, and it would really be horrible to lose access to them.
Brave should introduce its own userscript manager, perhaps by using Violentmonkey as a base, allowing brave users to have the full functionality of userscripts baked into the browser, and not reliant on v2 manifest.