There's a little feature flag command line trickery to toggle Manifest v2 which should still work on mainline 140+ onwards. According to the source code and Ge

There's a little feature flag command line trickery to toggle Manifest v2 which should still work on mainline 140+ onwards. According to the source code and Gerrit this hasn't been patched out yet: --disable-features=ExtensionManifestV2Unsupported,ExtensionManifestV2Disable