Swapfiets rental bikes have become popular in Berlin. They have a frame lock with an integrated chain. But even if you don't use the chain, what exactly is the theft scenario? The bikes are rather heavy—which I guess is intentional. Someone highly motivated could lift the bike into a van, drive home, and then worry about cutting off the lock. It just seems a bit far-fetched.
[0]: It was an ABUS like this: https://i.imgur.com/ytfVtex.jpeg