I think you are being nostalgic. The current generation of communication software (iMessage, WhatsApp, Discord, Slack, Signal, Messenger, Hangout, Matrix, etc) trumps what was popular in the 90s/00s.
And every one of them needs a separate client, with a separate list of friends and handles, a separate program to check and monitor, ensuring that they are well-behaving. The current generation of software has no equivalent of Trillian or Pidgin, which could interact across any protocol. Heck, the current generation has no concept of "protocols" at all, instead treating the network communication as something entirely internal and subservient to the program running it.
I know a lot of people hate it, but Signal piggybacking on your phone number and native contact list is a reasonable workaround to that “seperate contact list” issue. And mobile push notifications make that “a seperate program to check and monitor” a non problem, at least for me.
Did you check Franz/Ferdi? Though its Electron, it does support a lot of protocols.
People here (NL) barely use iMessage. They use WhatsApp mainly. I prefer (and do use mainly) Signal. For group chat, Matrix. Though Discord is (unfortunately) very popular. I dig it only for gaming.
Doesn't Ferdi just put everything into an embedded browser and call it a day? I may be wrong here as I used it for a brief stint, but that is basically what I picked up from it.
* single chat client (not a plethora on many platforms these days)
* ability to set online, offline, away, invisible (basically if you're available or not, whereas Facebook/Instagram/Slack don't really have that same separation and even the ones that do tend to not be as popular)
* Single-purpose platform, no other social media needed to carry it. i.e. Instagram's DM's are tucked away. Discord is technically single-purpose, but it's also at its core a social media platform, you need to join servers/Discords for all the separate things you're into, etc. so it's also not a single-purpose platform).
Nah not really. For normal users maybe, but if you were into programming you own utilities bots, scripts etc, today most platforms are very limited.
Out of all of them I like discord the most, but it's not hard to find problems even there. (weird scrolling, nearly useless search). I miss custom clients.
Plus, while I like the integration gifs images and some emojis, I generally find it overused in almost all servers I am on.