I'm a major Brian fan and it is a fairly poorly-kept secret that he's been very mentally checked out since 2021 or so. He's always been hit-and-miss live, but the tour he did with Chicago in 2022 was clearly a new low for him; he spent pretty much each show staring blankly into space at his piano and sang/played extremely little. Most of the fandom came away from it thinking he probably had dementia. There are some Reddit threads about his state based on that tour here:
https://www.reddit.com/r/thebeachboys/comments/vw5l7m/brian_wilson_elder_abuse/
https://www.reddit.com/r/thebeachboys/comments/vx0hug/requesting_recent_video_proof_from_brian_wilsons/
His daughters have since stopped him from performing, so I believe his latest public appearance was at the Grammys Beach Boys tribute last April. I've corresponded with some people who were also there and they said again he mostly just stared blankly while an assistant occasionally prompted him to wave and react to the show and such. Not really the sign of a well guy.
Anyway, given that this article is based on court documents I have no trouble believing it. That being said, I don't necessarily think he's prime deadpooling material, he actually looks much better physically in recent photos than he did in the mid-2010s when he was regularly battling ailments and seemed super frail. But yeah, don't expect him to have too many years left.