[masukomi]

Unless they paid for your relocation you don't owe them anything. MORALLY you definitely don't owe them anything. They lied. You agreed to do one thing... this is something else. You're only morally liable to do the work you agreed to.

At the very least you should talk to your manager / hr about this situation. Hopefully there's some other role that uses your skills that they can put you in. If not they need to know that this is BS and you're looking for a new job (NEVER BLUFF ABOUT LEAVING. They may call you on it). If nothing else this will, hopefully prevent them from doing it to someone else. Hiring someone is expensive, and the time cost of spinning up a new person is expensive. It's not in their best interests to lie to people. It costs them money.

On the other hand, legacy code isn't necessarily a bad thing. The work of bringing it up to modern standards can be rewarding (if they'll let you do it).