I finally got around to watching all of my season 1 DVD that includes the cast reunion with interviews from Meredith Viera and Al Roker. Mary's death of drug overdose was mentioned, as well as Nicholas sending in his remarks. But Susan was the conspicuous absence nothing was said about.

Susan Richardson is very mentally ill in real-life. Her health wouldn’t allow it.

https://news.amomama.com/278240-eight-is-enough-susan-richardson-became.html