Interesting that you mention a subscription service would more sustainable. I'm actually building a markdown note-taking app [1], that is based around a note-card format. We are currently developing a way to share a markdown card with a secure link.
All submissions would get a random ID, something like this example chosen at random:
https://markdownshare.com/view/b5abadcc-c0fb-473e-a34d-f27c0...
There were numerous bots who'd try to spider/probe the site collecting all the links. Handling millions of 404s a month wasn't much of a pain, because the service was pretty efficient, but it was just another source of annoyance.
Finally, and God only knows how, I'd get a ton of DCMA takedown notices for alleged copyright infringement. That's what pushed me over the edge into first of all making it read only, and secondly planning to kill it at the end of the year.
The same thing you see on random blogs, I don't think there's anything specific about my service in particular - just badly coded bots that don't have retry logic and blindly spam the same content to thousands of sites that accept comments, content, and pingbacks.
Unfortunately these kind of sites are an abuse magnet, so it is in the process of being retired.
I think any sustainable site/service like this has to be self-hosted, or come with a subscription. Because otherwise dealing with all the bots, spam-submissions, and other abuse is just too depressing.