2020-04-06. This informational bulletin explains Unscheduled Supplemental Services (including Flushing Services and Additives) and the Proper use of GM Simplified Maintenance Schedules. NHTSA's copy of the document (PDF)
Bulletins from the manufacturer that mention a maintenance schedule or a service interval, as NHTSA summarises them. They are not the schedule itself: read the document, and the owner's manual.
Position 8 of a VIN names the engine on many vehicles: how to read a VIN. Safety equipment depends on the trim, so decode a VIN to see what one vehicle has.