I don't think discord or slack is a great alternative as it is very ephemeral (and not really open source / decentralized) but I do think interacting with a mailing list is worse than a threaded forum type layout.
Email has never had explicit threading - email has a bunch of usage habits that many clients interpret as attempted threading (which often results in threads of steam receipts in my gmail webview FYI).
There are a number of clients that are particularly good at respecting the specific approaches to threading used by mailing lists and that come with a plethora of powerful keyboard interactions to make browsing a whole bunch of emails trivial. But newer devs may be quite unfamiliar with those options and I don't think it's particularly easy information to learn... especially if your webmail has "just worked" for most other uses.
The "In-Reply-To" header is described in rfc2822. It is an explicit header in the RFC that is how you create threads.
Every mail client I've used correctly understands how to thread reply-chains using In-Reply-To.
The thing you're talking about, steam receipts grouping, is not a feature of email, but a specific feature of gmail's web view which is not mandated by any RFC and indeed is not explicit threading...
But there is a real way to thread which is defined in the RFC, and if you use a reasonable email client (aka not gmail), every mailing list's threading will work for you.
You've been downvoted but I believe your post is accurate. At least with regards to IMAP and any RFCs I've read myself, though I'm open to hearing from others. Much of email is absolutely implemented in client-side 'de-facto' ways.
edit: I see now in the RFC that there is a description.
Discord is amazing, especially with the advanced search features, and now threads. An open source alternative to discord like the one featured on ShowHN recently would be ideal.
The only blocking issue is that their success (and subsequent spam/abuse issues) is greater than their focus on open access, to the extent that they formally disallow 3rd party client access.
2) throw the whole thing to the wind until I have better hardware
Hmm. You know come to think about it the chances are people with similar interests to myself are probably also stuck on older hardware too and probably won't be using Discord either...