That's one thing I'm currently missing in Ripcord (the Slack client). Or at least I haven't figured out how to do it. I use Slack from three devices, and sometimes I read a message I want to reply to later from a different device - marking messages as unread was a handy way for ensuring I don't lose them.

I wasn't aware of this Slack feature. I used reminders for that. Maybe that's supported in Zulip too.

Zulip lets you star messages though, and they show up in a separate inbox, so you can effectively bookmark things to come back to later.