(If you are not familiar with Matrix yet, please see for example https://joinmatrix.org/ for quick introduction)
Matrix is getting lot of traction in FOSS world and replacing or bridging various IRC, Discord and Slack chats into single interoperable and decentralized platform.
I'd like to suggest bridging the existing Node-RED Slack to Matrix and adding Matrix as officially supported way to join chats in the documentation. I'd like to emphasize that with bridging is not migration - those who are happy with Slack can keep using it like before and those who prefer a FOSS solution can join via Matrix.
A lot of large FOSS organizations like KDE, Gnome, Ansible and Mozilla have already done this so I think Node-RED community should follow.