Does it matter? Slack is free and most (all?) people I know only use the free edition. No different from using GitHub. People will always use free stuff if it's there and of good quality.
It's usually a good idea to lower friction and meet your users where they are. In the case of the React community, most have Discord accounts because Reactiflux was kicked off Slack for having too many non-commercial members. (Supporting open-source projects is an explicit non-goal for Slack.)
Discord welcomes open-source communities and is quite popular in the React community. If you are adding an open-source tool to the React ecosystem, those are both good reasons to prefer Discord. I certainly wouldn't want to build an open-source community on Slack after the way they handled Reactiflux.
I haven't used Discord but it says "It’s time to ditch Skype and TeamSpeak." Skype is primarily 1-1 and not for irc. I have not heard of teamspeak but a quick google suggests it's for gamers. I don't quiet get how it replaces slack if it's replacing skype.
edit: looked into discord closer now. discord has no pricing information whatsoever. thanks, i prefer something where i pay for and hope will last.
Discord welcomes open-source communities and is quite popular in the React community. If you are adding an open-source tool to the React ecosystem, those are both good reasons to prefer Discord. I certainly wouldn't want to build an open-source community on Slack after the way they handled Reactiflux.