I think his improvements in offense offset his defense being nerfed.
And don't get me wrong, he'd still be GOAT-level on defense. If Rudy Gobert and Kevin Garnett can have modern seasons (2004-05 onward) of defensive RAPM and defensive one-number metrics in that +5 to +7 per 100 possession range, Bill can too. Maybe even a little bit more because he had it all: no weaknesses, shot-blocking like Hakeem/D-Rob, and defensive rebounding like Rodman. Here's a great breakdown of his defense from the poster Dipper 13, who tracked hundreds of Bill Russell possessions:
Some might think a 7% block percentage and 36% defensive rebounding percentage at the same time isn't possible but Rudy Gobert has been close. 2020-21: 7% BLK and 33.5% DRB; 2021-22: 5.7% BLK and 36.3% DRB. Combined, that's 6.4% BLK and 34.9% DRB. Ben Wallace and Dwight Howard have some seasons/playoff runs that are relatively close.
And as for Bill's offense? I think +2 to +4 per 100 possession kind of impact is possible. Imagine a prime Deandre Jordan roll-man game with 200-250+ dunks per season, plus the ball handling and playmaking of guys like Draymond Green, Joakim Noah, Giannis Antetokoumpo, and Domantas Sabonis.
11-13 highly efficient roll-man points from PnR finishes, cuts, post-up seals, and offensive rebounds. The added dynamics of the ball handling and passing open up alternative creation pathways in transition and in the short roll and hand-off actions.
But the big swing skill would be face-up slashing in the half-court setting. In isolation and as a PnR ball handler. Think Giannis. Most players need a perimeter shot to make this work but when you're a big man and have a freakish combination of size, athleticism, and ball handling, you can be an exception. You can force the issue and get to the rim. Today's three-point shooting, spaced-out defense style of play, and dribbling, carrying, and traveling rules have made that far easier to accomplish.
All up, I could see his offensive numbers bleeding out to something between 16-24 ppg and 6-8 apg with 55-60% FG and TS% ranging from 58-62. Add that onto 14-17 rpg, 1.5-2.0 spg, and 3-4 bpg and you've got a defensively-slanted two-way all-time great.
I even think there's an argument his free-throw shooting could improve a little due to equipment advancements. Standardized rims, backboards, controlled arena temperature, and a game ball with eight panels and better grip. Fun fact: league-wide FT% went up when they switched to the eight-panel ball. I'll give Russ 60% FT at best. He shot 56.8% FT across his career. 77.8 FT+. Adjusted for today's league average free-throw shooting (78.4) that's 60.9% FT.