Matrix (https://matrix.org https://matrix.org) is a good alternative to both Slack and Discord. The most complete client implementation is Riot (https://riot.im

Matrix (https://matrix.org https://matrix.org) is a good alternative to both Slack and Discord. The most complete client implementation is Riot (https://riot.im/ https://riot.im/). The protocol itself is federated, so you can communicate with other Matrix users from your self-hosted instance. There are also bridges to IRC, XMPP, even Slack..