URL bar lock icon says connection is not secure when on "view-source:[...].onion" URLs
On regular https websites, if you go to view-source:https://the-url.lol, the icon in the URL bar is an encircled letter i. If you click on it it says in green "Connection secure", and if you click the right arrow it says "You are securely connected to this site".
On onion sites, if you go to view-source:http://the-url.onion, the icon is a lock with a red line through it. If you click it says "Connection secure" again. If you click the right arrow it says "You are securely connected to this site", just like https except underneath it says "Connection is not secure".
v2/v3, it's the same.