Like LAMP for Slack, Discord also used MongoDB prototyping in the first few years, but while they prototyping they prototype in a manner they can easily migrate to Cassandra later. Then they successfully migrated to Cassandra. This is brilliant.
I remember the React community migrated from Slack to Discord, I guess one of the reason could be the performance. According to the blog, at that time the team only have "14 people full time", which means they've got a very high velocity, amazing.
I didn't find any summary of their whole tech stack but there are several insightful articles from their blog like.
We got kicked off Slack because we got up towards 10K users signed up, and Slack locked us from having any more people join. Clearly, we weren't a good fit for their business model. Worked out well for us - we've loved Discord ever since we switched over!
Every time I use Slack I found the business model is not as generous as Elixir - there are too many constraints for free users/organizations.
I'm pretty sure the Reactiflux channel exceeded 50k long time ago.
Looking at the server right this second, I see 2800 online, and 72.5K total members.