On the Dronecode contribute page https://www.dronecode.org/contribute/, there are multiple references to the PX4 Slack channel, but the links take you to the discuss forum instead of the Slack channel. That page doesn't seem to editable in Github so I'm putting the suggestion to change it here. Hope it helps!
Thank you very much - you're right, it isn't editable. I have fixed to point to the PX4 Discussion Channels section of the devguide (this points to slack, discussion board)
You're welcome! Glad I could help!