Matrix

Chat systems aren't email, or Usenet, or forums, and while it is a good feature in the context of those async / longer-form communication, where you need the context, it doesn't work nearly as well for realtime chat, where you already have the context because it happened two seconds ago.

The convention of replying in thread or in channel is a combination of personal preference (I like/dislike or am/am not used to threads), group expectations (we have agreed to reply in threads/in channel), and muscle memory (I mostly talk in channels that reply in thread, but this one expects it in channel). As the number of participants increases, it gets hard to manage, so you get a mix of in-thread or in-channel replies (and in-thread replies to in-channel replies), which leads to a fragmented, inconsistent mess and people complaining about both styles at once.