Delta Airlines has modified its seating algorithm to selectively block serveral rows of seats that will only be visible on seating charts to families and groups booking together, making it easier to keep the group together.
Solo passengers or groups of two won’t see those seats on the chart when picking seats, although they will be visible to high-status elites even if alone. The number of rows blocked depends on the flight and expected passenger mix, generally ranging from two to five rows.
The software will monitor the seat assignments to determine how the plane is filling, and can release seats if fewer groups and more solos book the flight. Most of the blocked rows are near the rear of the plane, often a preferred choice for families.