My company wants me to add new features to their existing cappy system when I'm on my notice period...

The same happened to me. When I resigned they forced me to use 2 weeks of vacation they didn’t let me take because they didn’t want to pay them as extra.

I actually wanted to use them not to see them during notice period.

But on the last day of job/forced vacation, they called me for a not better identified bug.

Well I turned my phone off. I take my vacation seriously.

Technically the notice is still counted as your employment. Granted, your work should be ramping down but it's still work. If they want you to work full steam ahead up to 5pm on your last day it's within their rights and quite frankly it's your professional duty.

However, if they ask you to do something unethical, immoral or illegal or they are being abusive, asking for overtime... then just walk out and never return.

Having said that, adding features to crappy code feels unethical and abusive 🤣

I’m not seeing what’s wrong with this.