It was long before that... I have a schedule from 1994 in front of me that shows one AM outbound and one PM inbound running over the Wildcat. Not sure how much further in the past it started, but at least 1994. By the 1998 schedule there were two outbounds and three inbounds over the Wildcat, which is the case today. The Downeaster started running in 2001, with four round trips all running over the Wildcat, and that was increased to five in 2007.
And then there's deadheads...
[edit] Looking at that very useful
"Changes to Transit Service in the MBTA District" article published in
Rollsign and formerly on the NETransit site (via Web Archive), it states that with the resumption of through commuter service to Haverhill on December 17, 1979, "Although most trains to Haverhill use the Western route entirely, some reverse-commute trips in each direction use the Wildcat Branch and the New Hampshire Main Line to travel from Haverhill to Boston."
1979!!! Waaaaay before the Downeaster, that!!