Encountered this today where a friend posted a link he'd had shared with him in an entirely other chat, where I could allow messaging from the original poster that I'd never met.
I'm assuming this is vaguely online safety act related and I'm pleased to see it has immediately made things far worse.
Security alert: YouTube added a feature that now puts your username in shared links, so if you share a YouTube link on here, everyone who clicks it will know your YouTube username.
To turn it off, go to Settings - Privacy and turn off "Channel visibility for shared links"
