If you leave a server, you need to be invited back. Where, with slack, you can remove it from your team list and read it whenever you please.
Another issue with Discord is, members won't always have the same names they do on the forum. For example, I'm part of quite a few servers, including my own, but my name wouldn't match my forum name. You can change it, but whatever you name is on server, it's the same for every server you join.