That's a great question! One of the things I enjoyed during my time at Slack was their willingness to contribute to open source projects. We had similar IP clauses, but asking permission to open source things was straightforward.

The most important concern (IMO), was considering whether we could commit to properly maintaining a project. Before open sourcing anything, you need to discuss how you'll go about managing an issue and pull request backlog, so that people don't come across "dead" projects under your stewardship.

In a high growth startup, I do think something like this could happen again, but as a company grows, there are certainly more layers that can make it difficult to share things openly.