This got passed around as a common fact in the wake of the 2008 financial crisis. Wages from the early 70s through 2010 or so were flat (not negative, but flat) due to inflation. Things have shifted since then.

https://fred.stlouisfed.org/series/LES1252881600Q

Note that the graph shows median wage; it isn't as affected by a few high earners as average wage would be. The 2010s were a period of relatively low inflation and wages had a chance to catch up a bit.

What is true is that productivity has leaped massively since the 70s, but median wages have only crept up somewhat. The argument needs to shift to be around how the working class was screwed out of their share of productivity improvements. That's not likely to change until we have more unions and overall something closer to Socialism.