Yes, the 90 minute number is in the Contract of Carriage and has been there for many years. This site gives a good summary
of the conditions which qualify for a refund -- https://pro.delta.com/content/agency...y.html#refunds

Non-Refundable Ticket

Travel agencies can refund a non-refundable ticket(s) when there has been a Delta schedule change if one or more of the following apply:

• There is a change to an itinerary resulting in no comparable schedule being available within 90 minutes of the originally scheduled arrival
  
• A change in the routing of a scheduled Delta flight which adds one or more stops to the original itinerary.
  
• A change in equipment from DL mainline to DL Connections carrier
  
• Any change that causes a misconnect

PNR must be documented with the following OSI entry:

OSI DL REFUNDED PER UNACCEPTABLE SKED CHG/ DATE (impacted flight date)