“We decided early on to store all chat history forever so users can come back at any time and have their data available on any device.”
Discord started on a single MongoDB replica set and outgrew it at 100 million messages. The fix was Cassandra and a careful partition scheme. The engineering is good. The decision that matters is the first one: keep every message forever. Your 2016 group chat is still on their servers, searchable by anyone with a subpoena.