As an open source developer, I would refuse to "reach" my users in such a tacky way as selling ad space in my README. You're correct that the README is one of the few touch points with users, and I wouldn't dare waste that on something I consider so disrespectful.
Oh, I'm not talking about selling ad space. I agree that what Funded did was misguided at best. But what my parent was responding to was the idea of even putting a little ad for a paid product by the same developer or company.
If JetBrains, for example, wanted to remind me on the README for Kotlin that they have an IDE that works well with the language, that doesn't offend me.
It's about as relevant as advertising gets, and it's not coming from a pool of clickbait ads. It's a developer using their open source work to generate interest in the work that pays for it.