I’d imagine that Brian Wilson was declared “not competent” at some point in his adult life while he was still recording albums, signing contracts, and even performing.
Sly Stone may have a similar situation.
I remember he we was in violation of Paragraph 12, under Section 936A of the California Catch a Wave Statute: failing to surf, neglecting to use a state beach for surfing purposes, and otherwise avoiding surfboards, surfing and surf.
Sly was living in an old van behind a building last I heard.
Brian’s mental problems began in the ‘60s, but I don’t believe that he was under conservatorship until circa 1991 when his cousin Stan Love (father of the NBA’s Kevin Love) sought to become his conservator. That was during the period when Brian was being controlled and overmedicated by his crackpot psychologist Gene Landy. Brian didn’t want Love as his conservator but he agreed to a neutral 3rd party. When Brian got remarried, I think his wife became his conservator and eventually they got out from under the court system. But, yes, there absolutely was a period c. 1991-97 when Brian was under conservatorship but still performing, recording music, etc.
It's a well known fact that Brian had mental problems. Unfortunately, he came upon a Doctor who took total control of his life while keeping him drug induced. That lasted for years, until a later girl-friend was able to convince his brothers about the slick unscrupulous Doctor.
She knew it for some time, but didn't have solid evidence until Brian's housekeeper supplied physical evidence to the girlfriend. The brothers immediately corrected the situation after being in denial about the Doctor for years. Correct me if I'm wrong.