I agree with @Gayrat, I stoped looking at the chat now for some time now, I rely only in the Forum, why do we need a chat?
But if we need a chat, I agree with @huna, why not Matrix to be more in line with the open source comunity?
Both apps are looking to be bought soon (Salesforce for Slack and Microsoft for Discord) and they may change stuff to the chat to integrate better with the comercial interests of their respective companies (integrating it to XBox live or wathever) this is why they are buying it.
So maybe we are taking a decition in moving grounds, and 1 year from now Discord puts a paywall for bigger comunities or something, same for Slack, you just can’t tell.
Hopefully we can take control because we have amazing open source projects that make running a server easier, whether is a chat or an API 
Not all comunities have the skills to run their own server, I don’t think that is the case for Strapi.
I get that is extra work, and I like the approach of Strapi to not be hard core open source everything, and take the most efficient rout when possible. But maybe this time (like with the Forum) is a good decition to take the future in your own hands, and not rely on this big companies that will very likely change a lot in the comming years.