I was reading Bleach at the time and I never saw any mention in the news of health issues. Usually Jump will give their artists breaks to recuperate, like with Hunter✕Hunter. If there was any announcement about health issues afterwards, it was probably to save face, but I never saw them. All I know is that Jump made a big announcement saying that Bleach would be ending after the current arc (it's dragging and we want it finished), it was expected to be eight months (iirc), Kubo kept up his extremely tiring counterattack-counterattack-counterattack-counterattack-counterattack-counterattack-counterattack... schtick the entire time, and then out of the blue in an issue of Jump there was a small announcement that Bleach would be ending in two weeks. Just like Samurai 8's two week ending announcement.
You can infer conspiracy theories from sales charts, but if your numbers drop enough in Jump and don't recover, you're gone. There's thousands of other manga artists fighting to get your spot, and that's how it's been for 50 years.