Some information here about 'post April' seat blocking -
https://news.delta.com/delta-expands...s-all-aircraft
Looks more like a way to control overbooking by taking some seats out of inventory.
For example instead of overbooking for ~160 seats on a 737, they will only allow overbooking for ~130 seats and some/all of those blocked seats can be opened up if overall sold tickets on that flight are in 130-160 range.
Seat blocks are not lifted if sold tickets are less then 160 and as blocked they are not available for upgrades.