I would personally draw the line a the data centres. If your infrastructure involves data centres, this is not self hosting. In my opinion.

I think it's a pretty clear line. Self hosted means you can run it locally in your home, using a saas platform or vps is just a convenience, not a requirement.

I think the line is obvious... If you can't unplug it, you're not self-hosting.

You should have the option to host it at home. Then you may choose to host in a vps. Must be vendor agnostic.

IMO physical hardware in your own material possession, so only the last of your examples.

https://en.wikipedia.org/wiki/Self-hosting_(network)