Did you see Zulip? I heard it from some students.

Yup, i use Zulip at my job, we were on HipChat that recently shutdown due to acquisition by slack. Zulip is exactly like slack with one added feature of having topics with streams(channels on slack). Plus they have a fully free open source hosted version with all features. Their cloud one is the one you have to pay for which is similar pricing to slack.

what about the bots?

Hmm .. Zulip. Surely check it out 👍🏼 But as Rushal mentioned, I think the bots are the killer functionality in this community. For eg. there are bots right now which are helpful in running initiatives like #buddy-calls and #ri-trailblazer

There is good amount of customization and ability to add bots. https://zulipchat.com/help/add-a-bot-or-integration