Not OP but I think it's acceptable. I'm happy to see my favourite free (as in freedom) projects' developers being supported. If someone doesn't like it and if it's truly free software then anyone can fork it to remove that ad and we don't have a problem. That's how free software works, if you can't, then it's not free software. If one just won't fork and opposes what a free software project does then that's just entitlement.
In the d3 example I tried to exaggerate the hypothetical intrusiveness of ads by embedding it in all the js source files.
In your specific case, if nobody is complaining about your ad inside of a single "README.md", I'm going to presume it's "acceptable".
I imagine the latter comes with a link, perhaps even a tracking one, to sign up for the service.
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.
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.
Tracking the campaign? Yes, probably. But tracking you? How do you expect that to work?
Making ads contextual to the place they are and tracking the campaigns to discover what works and what doesn't is how it should be done.
Although:
> decides to embed advertising such as "Try LINODE for 30 days!" in "README.md"
Are you trying to say that this is also not acceptable? Our company sponsors a few open source npm projects, and all of the projects we sponsor have our logo in the README.md - is that really not socially acceptable?