https://www.autocontrol.app/force-enable-Manifest-V2-support
This action could be reverted by Remove Manifest V2 availability.reg + steps 2, 3.
Go to chrome://policy and click on the Reload policies button.
Go to chrome://flags/
Run browser with shortcut parameter (add to Target field in details)
-disable-features=ExtensionManifestV2Unsupported,ExtensionManifestV2Disabled
It seems not required in Chrome 138, but 140+ (dev/canary) does not allow enabling mv2 extensions without this command.