As of my last update, Delta Airlines allows passengers to select seats during the booking process or at any time before their flight through the airline's website, mobile app, or by contacting Delta customer service. However, specific policies regarding seat selection may vary based on factors such as fare class, frequent flier status, and availability.
Here are some general points about Delta Airlines' seat selection policy:

Seat Selection Fees: Delta typically offers complimentary seat selection for passengers booking standard economy fares. However, for Basic Economy fares, seat selection may either not be allowed or may incur an additional fee. Passengers booking higher fare classes or holding elite status in Delta's SkyMiles program may also enjoy complimentary or discounted seat selection.

Advance Seat Selection: Passengers are usually able to select seats during the booking process or at any time before their flight, subject to availability. It's advisable to select seats as early as possible to secure preferred seating arrangements, especially for families or groups traveling together.