If you bought the iPhone from Apple in Germany with the 24-month financing option that included the upgrade offer, the option hasn’t disappeared.
For iPhones bought before July 22, 2026, Apple says the original offer still applies: after 12–13 monthly payments, an eligible iPhone in good condition can receive a guaranteed trade-in value of at least 50% of its original purchase price toward the remaining financing balance.
I’d sign in to the Apple Store and check Account > Your Devices. Apple says that’s where you can see whether the iPhone is currently eligible for an upgrade.
The important part is whether your original purchase actually included the upgrade option, so I’d also check the financing agreement/order confirmation from when you bought it.