> I've never open sourced it though because I guess it's a bit of a grey area

I'm maintaining the TT-RSS plugin feediron https://github.com/feediron/ttrss_plugin-feediron that fetches full-text data, so my thinking is this:

At the end of the day if it's a openly available website and you are personally (through your own server) fetching the resources I don't think anyone has a right to complain.

Now if you were offering it as a service it might arguably be a bit more grey, but only if you're ignoring the robots.txt file