Good notes has an option to revert to v5 and I haven’t had any issues so far staying on v5.
I thought they also had a one time purchase option for v6 but it’s been awhile since I looked.
They did the switch better then notability tried to do. Notability tried to switch otp users to their new plane after a grace period of a year. They caved to backlash and added a legacy plan for older purchasers.