Today, it would be unthinkable for an NFL game to not be televised. So when was the last time an NFL regular season game did not appear on TV at all? According to the historical NFL TV research at 506sports, the answer is 1975 (when it happened twice).
The last untelevised NFL game was on Saturday 11/1/1975 when the Giants hosted the Chargers at 1 pm. Why was there an NFL game on a mid-season Saturday? The Giants shared Shea Stadium with the Jets that year as their new stadium in the Meadowlands was under construction. Shea was unavailable to the NFL until the MLB season ended as the Mets were the primary tenant. This forced the Giants and Jets to play on the road the first two weeks of the season. To squeeze in 14 home games at Shea over 12 weeks, the NFL scheduled the Giants for a few Saturday home dates. For games like this played outside of the normal network TV windows, the NFL allowed the road team to sell the TV rights to a local station (same thing for the home team if the game was a sellout). But no local station opted to pick up this game so it was not on TV.
So when was the last Sunday afternoon NFL game which went untelevised? That occurred just a few weeks earlier when the Patriots played at Cincinnati on Sunday 10/12/1975 at 1 pm. Why did TV shun that game? Well, NBC aired game 2 of the Cincinnati-Boston World Series that day at 1 pm. The same option for teams to sell the local TV rights applied in cases like this when NBC carried a Sunday baseball postseason game, All of the early afternoon NFL games bumped from NBC that day were televised by a local station except for NE-Cin. With the World Series featuring teams from the same TV markets, no local station wanted to televise it and compete with baseball. The idea that baseball would render an NFL game to not be televised seems hard to believe now, but it happened in 1975.