The cutoff doesn't tell us when the requirement began. The current cutoff is 1997, which is when the current version of the reg (which separated complex and HP) took effect. The original high performance endorsement (which included what is now both complex and HP) goes back further than that.

I don't know when it all started but I have a copy of the pre-1997 version of Part 61. The "cutoff" date for the HP endorsement in that version is November 1, 1973. If they wrote the cutoff the same way then as now, then it suggests there was a HP endorsement requirement in 1973. And I know of at least one FAA Chief Counsel opinion from 1980 that refers to it, so it must have already been in existence.