The closest thing I’ve found to passive income is royalties from books I’ve published to amazon, but even then the books need to be promoted otherwise they’ll lose their ranking.
Second best is affiliate marketing for products that have a monthly subscription. I don’t know why I didn’t start with that sooner.
Yeah back in the early days of SEO, I used to rock the greyish hat SEO and did quite well for affiliate marketing.
But whenever you do great hat or black hat, it never lasts.
I’m getting back into it though but have to be patient with it.