Slack started as a set of features over IRC (at least to a large extent), and was compatible with IRC servers for quite a while. Turns out, when you try to make

Slack started as a set of features over IRC (at least to a large extent), and was compatible with IRC servers for quite a while. Turns out, when you try to make IRC actually suitable to the environments where users aren't always on (among other deficiencies in it for enterprise use-cases), you end up with something sufficiently customized to no longer be IRC. You can either lie and say it is (but get called it), try to change what IRC is for everyone else (good luck, the open servers don't want your features and consider lack of history a feature not a bug, but it is a "bug" from an enterprise perspective), or create something new out of the old system. Then you get Slack again. Or you can make a largely from-scratch system (Discord) that resolves these problems from the start rather than trying to patch over something that doesn't actually do what you need. Or you can try to make a new protocol (XMPP, Matrix) that tries to resolve these and hope people build compatible clients and servers and don't fork it in a way that breaks compatibility.