At least Brave and Vivaldi both use the Chrome Web Store, which presumably will no longer allow such extensions. So either they'll have to build their own alter

At least Brave and Vivaldi both use the Chrome Web Store, which presumably will no longer allow such extensions. So either they'll have to build their own alternative, or change the browser to allow out-of-store installation, and the extension author(s) will have to support that. So it's possible, but not immediate.