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.
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.
edit: I see now in the RFC that there is a description.
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.
So at the one end of the spectrum with have IMAP/SMTP, in the middle we have things like Telegram (everything's GPL3! The protocol is terrible and here are all the details! Go nuts!)... and at the other end we have Discord: One Protocol, One Allowed Client™®©.
Do I use Discord? No.
Would I like to use Discord? Yes.
Why can't I use Discord? The client is too much for my older laptop.
Would I like to use Discord? Yes there are a pile of people on there I'm missing out
Can I do anything about it? Yes, I have two options:
1) use https://cancel.fm/ripcord/ until it suffers the same fate as https://github.com/Bios-Marcel/cordless
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...