I kinda feel like that's more due to lack of interest than anything else, at least on the part of WhatsApp, which is restricted enough feature-wise that it'd be somewhat feasible to reverse engineer and build a client.
If I think about something like Slack or Discord, though, they surface area of those things are just huge. So many protocol details to work out, and so many features to duplicate for a third-party client. And they're such moving targets, too, with features added at a regular cadence (unlike AIM, ICQ, etc. back then, though even then Gaim/Pidgin didn't have voice & video support for a long time).
I also think the third-party client drive for AIM and ICQ was driven by people who wanted to use it on Linux but had no way to do so at all. The clients were Windows- (and Mac-?) only, WINE was a pain to use back then, and there were no web clients initially. But with Slack and Discord the impetus just isn't there. At least with Slack (I'm not super familiar with Discord), Windows, Mac, Linux, Android, and iOS are all officially supported, and there's always the web client if you don't like the Electron-based desktop clients.
On top of that, Slack's API is rich enough to enable things like a Matrix bridge, so, again, probably just not enough interest to do something truly standalone.
The only thing I'm somewhat surprised about is that no one has tried to build a third-party standalone WhatsApp client that doesn't require being tethered to the mobile client. But perhaps Facebook/WhatsApp has done a better job at obfuscating and hiding their protocol than AOL and Mirabilis did long ago. Or maybe the interest just isn't strong enough to warrant the effort.
Interesting, what makes you think the surface area of Slack is larger than that of WhatsApp, for example?
I think the latter hides a lot of complexity behind a simple UI, most notably all the E2E encryption logic and the purely client-side message storage, with an optional backup system and even its own built-in remote access server (for the web client).
Compared to that, Slack is mostly a HTML renderer as far as I can tell, with most of the complexity server-side (e.g. complex notification rules, multi-client support etc.)
I suppose the poster above you is referring to user experience - WhatsApp doesn't even have reactions, never mind the huge list of plugins that slack has. The only thing in favour of WhatsApp as a user is the encryption, which AFAIK slack just doesn't bother with (maybe that changed now).
I‘m still not convinced that all of these user-facing features necessarily translate into a larger client codebase.
My guess is that WhatsApp (and Signal, Matrix etc.) client logic is much more complex, since so many things that are fairly easy to do server-side need to be reliably performed in the client instead once E2E encryption is introduced.
Same here. I'm literally forced to use it with no more than a dozen contacts among close friends and part of the family, and despite the very short list of people, it's absolutely impossible to convince anyone of them to use anything else.
I would so welcome a FOSS client.
If I think about something like Slack or Discord, though, they surface area of those things are just huge. So many protocol details to work out, and so many features to duplicate for a third-party client. And they're such moving targets, too, with features added at a regular cadence (unlike AIM, ICQ, etc. back then, though even then Gaim/Pidgin didn't have voice & video support for a long time).
I also think the third-party client drive for AIM and ICQ was driven by people who wanted to use it on Linux but had no way to do so at all. The clients were Windows- (and Mac-?) only, WINE was a pain to use back then, and there were no web clients initially. But with Slack and Discord the impetus just isn't there. At least with Slack (I'm not super familiar with Discord), Windows, Mac, Linux, Android, and iOS are all officially supported, and there's always the web client if you don't like the Electron-based desktop clients.
On top of that, Slack's API is rich enough to enable things like a Matrix bridge, so, again, probably just not enough interest to do something truly standalone.
The only thing I'm somewhat surprised about is that no one has tried to build a third-party standalone WhatsApp client that doesn't require being tethered to the mobile client. But perhaps Facebook/WhatsApp has done a better job at obfuscating and hiding their protocol than AOL and Mirabilis did long ago. Or maybe the interest just isn't strong enough to warrant the effort.