In case Discord implodes:

• Forums: https://forum.neverwintervault.org/ - lacks real time chat
• IRC - text only, bit of a barrier for keeping history, less features
• matrix/element - https://element.io/en - chat based
• Zulip - https://zulip.com/ - free for open source projects or can be self hosted

Things that would be useful:

• Histories
• Threads
• Image/file uploads
• DMs
• Moderation tools
• Easy to access
• Bots for some functions

Related: https://github.com/Tyrrrz/DiscordChatExporter