Parties can have exceptions and limitations to the rights from Berne, so it is not necessarily illegal in all of them, but I'd expect it is in a large fraction of them.
[1] https://en.wikipedia.org/wiki/Berne_Convention#/media/File:B...
* you own the rights to,
* is in the public domain,
* constitutes fair use, or
* you have consent of the copyright holder."
They claim their site is for the above, and people can submit a notice to have content removed that doesn't fit that description. However, the reality is they want people to post and share content from other sites, and they'll happily remove their advertising, and add their own.
I searched site:outline.com on Google, because I wanted to see for myself what people are sharing, and if I might be mistaken. The first result was this article...
https://www.outline.com/vF4aGL
The source is The Free Thought Project. Hmm, I thought maybe this is some type of public domain news source, so I visit them. Nope, they're not. When I visited The Free Thought Project, I instantly get a popup saying, "Thanks to a coordinated attack on independent media by the establishment, this website is at risk of shutting down due to our ads being pulled and our social media pages entirely deleted. We need your help to survive, click below to see how you can make sure we stay around." This is the first Google result for an Outline article, and the company being stolen from has a popup that literally says they're struggling to stay afloat.
I haven't previously heard of Outline, but that is definitely a disgusting business model.