I had a _really_ in-depth conversation with a friend with multiple interesting lines of thought going at once that only worked because the chat we were using had a per-message reply feature. The conversation would not have been as meaningful to me otherwise. I'm not sure IRC could have supported a chat like that...

(admittedly I haven't tried anything other than a few basic IRC chats, never really getting deep into IRCv3 or new features and stuff)

In IRC you'd just paste the relevant message or part of a message you're replying to after a greater than sign and then post your thought in the following message. Works totally fine. I have used IRC, XMPP, and Matrix to chat with some of the same close friends, I don't find actual replies to be necessary, and I think they kinda disrupt the flow when used too much.

I can appreciate that per message replies are handy in modern messages. But if you were having _such_ an in-depth conversation at the peak of IRC, you may have moved to email to make it async and allow for more fully thought-out replies.

Email is still a great way to communicate with people today.