Speaking of hosting, Discord used to be one of the biggest (inadvertent) image hosts, so they might have set up the system to reduce legal exposure than to monitor conversations per se.[1]

A lot of the internet broke the day they flipped that switch off.

Weren't external Tumblr hotlinks also a thing back in the day?

[1]: https://www.reddit.com/r/discordapp/comments/16uy0an/not_sur...