What year did Federal law state spouses automatically become beneficiary of 401k plan?

I believe that spouses have been the automatic primary beneficiary of pensions and 401(k)'s for many years - at least since the early 90's and perhaps longer. If you are married and want to name someone else as the primary beneficiary your spouse must sign a written waiver.

The Employee Retirement Income Security Act of 1974 (ERISA)