It is not clear if this is post vaxx when the change started. He is a long time vegan so it could be one or both of those factors at play. Nevertheless, in a recent article they quote his daughter as saying:
In another post, she wrote, “Some of us do think he’s mentally ill or experiencing something like dementia. He really acts like he hates and does not know his children frequently.” She added
https://www.msn.com/en-us/music/other/what-s-going-on-with-russell-simmons/ar-AA1cQXrf
...more likely Rick Rubins' bottom.