A Googler on the commit explains: "MV2 extensions are no longer allowed in any supported version of Chrome, and we are removing support for them and the associated functionality. We won't be able to provide / maintain this functionality indefinitely due to the complexity and tech debt, as well as the security risks it entails (we've actually found a number of bugs that are specific to MV2 lately). Of course, other browsers can continue supporting these if they so desire."

This will also impact other Chromium-based browsers, though the comment notes that "other browsers can continue supporting these if they so desire." Neowin points out that Microsoft Edge and Opera are likely to follow suit. Chrome 150, set to be released later this month, will remove this flag, while other leftover bits of Manifest V2 will be removed in the v151 release.