Delta's FF program was still giving 1000 mile minimum miles in Feb. of 1995 along with the extra 1000 when you upgraded to F. The hotels in Delta's program still gave 1000 miles at that time too.
USAir in November 1990 was crediting 750 miles minimum. The first 500 minimum I could find in my old FF statements was UAL which began their downgrade to a 500 mile minimum on Jan. 1 1989.