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.
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.