Tanking has presumably always existed. It's hard to believe that teams in the 60s, 70s, and 80s didn't realize that putting their worst players on the field towards the end of a lost season would do a lot to help them from repeating the same fate the following year. The incentive is obvious.
It does seem more recent, though — the advent of a trend where fans are actively rooting for their favorite teams to lose games as soon as they nope out of playoff contention. Whether it was the brazen rebuild of the NBA's Philadelphia 76ers — dubbed "The Process" — or just a shift in conventional wisdom in the social media era, it's hard to find fans who want to see their team win once the losses have piled up to a certain point.