The transition from Manifest V2 to Manifest V3 has been underway for at least five years. It makes new APIs available to browser extensions and removes old ones. The most salient of these is the blocking version of the webRequest API, which is used to intercept and alter network traffic prior to display. Under Manifest V2, it's what extension developers use to stop adverts, trackers, and other content appearing on pages, and prevent certain scripts from running.