whats the lowest battery DOD allowed

Remaining charge and battery health are two different measures. Remaining charge just lets you know how much charge is left and would be a good indicator of when you need to charge your phone. You can use your iPhone if the battery charge remaining is 80% or below. Though it's not advisable to let the remaining charge go below 20% or ever let the battery go to 0.

Battery Health is an indicator of how much "life" is left in the battery and when Battery Health declines to 80% or less it's time to invest in a new battery.

If you are asking about Battery health, then yes, below 80% it would be recommended to pay Apple to replace the battery in your iPhone.