It’s the level that behavior has been amplified that’s scary and unusual, before the internet it was only the most extreme fringe groups. It isn’t normal to go online and feel like a 50/50 chance everybody you meet is an extremist. That’s a recent thing just sometime within the last 10 years and it’s never been like that before ever
It doesn’t meet every requirement you listed but privatebin.info supports file attachments, configurable expirey, and burn-on-read, and is fully end to end encrypted