It was about 10 years ago. First, they made the switch from using per48 minutes to the advanced stats. At this point, Russell & Wilt had something like 50-60% creb% (Like Russell had seasons where he was literally 12.5% orb & 43% drb) and 6-10% blk%. Then they "got new data" *cough* ******* bullshit *cough* that brought all of their already made up numbers down.
Defense was tied to drb% for whatever reason, so literally anybody with good drb% had high defense, which capped at 96 (again, for whatever the **** reason). So when a guy like Troy Murphy's 08-09 season came out, he had 30% drb% and 96 defense. Literally as good of a defender as the great Bill Russell. He was also 100% at SF, but that's neither here nor there. After a lot of ********, they took drb% out of the defense algorithm & changed a few things, and suddenly, Bill was in the 70s every season. This is 100% tied to blk% & stl%, two stats that they completely fabricated for the time period. Both Russell & Chamberlain should be 90+ defense every season from the footage I've seen. You could literally double the number of blocks estimated fabricated, double the %s and it would probably still be low relative to what they did and their actual impact.
tl;dr: When they took drb% out of the algorithm, Russell's D plummeted due to absurdly low-balled, fabricated numbers, and when it was part of the algorithm, his D was absurdly not any better than people like Troy Murphy's & Reggie Evans because everybody had great defense.
tl;drthetl;dr: WiS made a bunch of **** up & looks ******* stupid because of it.