All the free-hosting sites I look at have a TOS gotcha saying something like "Files must be available for viewing on an
active website" (italics mine!). None of them ever say what criteria apply to make a site "active".

I have tried a few sites. I go through the labor of installing hundreds of pictures and web pages. Two months after activation, my site is GONE with no warning or explanation.
Before this happens to me again, can you please explain how you tell the difference between a "file storage" use and an "active site" use? Any guidance will be appreciated.