The exact same happened with the draft feature. The importance of habit forming UI is very well documented, going back at least as far as Jef Raskin's The Humane Interface and the draft feature completely breaks it -- if you as much as accidentally leave a character in there, you can't find the channel/DM at its normal place. Despite repeated calls to make it optional, nothing.
Threads are also broken, has always been. For example, if you get a notification in mail and it's in a thread, you can't jump to it, clicking the link dumps you into the channel and you are left wondering. Previously (as in, going back at least to '90 or so with IRC) you were able to skim the entirety of discussion in backscroll, now you'd need to open every thread on its own. As a senior developer, this was tremendously helpful to everyone because I could just skim a larger batch of discussions at a time and give some advice later as needed. Both of these problems could be solved if threads could be opened, you know, thread style into the channel. But... no. Also, if you answer in the special "threads" view then you will need to click the new answers link every time someone answers. It's terrible UX.
It used to be that adding a text snippet was right in the "add" menu you opened on the right hand side of the text, now it's in a submenu, seriously slowing down creating one. The menu wasn't getting large at all, no reason to do this.
It's very strange, they are the top dog, with immense inertia but it doesn't mean a younger, more eager, better UX, slimmer chat won't replace them. Digg to Reddit, remember? Until then, does anyone want to write a "Better Slack" collection of scripts injected into the app? I pledge 20 USD to fix these three problems, anyone else?