Yes. As of a couple years ago at least, Slack was a monolithic PHP app that could support a few thousand users in a single team. Each “workspace” was assigned to a server and scaled vertically. LinkedIn notoriously had two Slack workspaces in order to support all of their users, and I’m guessing other large companies have had to do the same.

I’m sure Slack is using their vast resources and large team to solve these problems at this point, but it probably hasn’t been easy to create a horizontally scalable system from the early codebase, especially when the data model was designed for a single team of users.

I would assume so as well. Maybe a Slack engineer can chime in?