Sure, not many, if any, clients handle an arbitrary list of message id's there, but it's clear[3] that's what it's for.
[1]: https://www.rfc-editor.org/rfc/rfc822#section-4.6.2
[2]: https://cr.yp.to/immhf/thread.html
[3]: https://www.rfc-editor.org/rfc/rfc5322#section-3.6.4
I'll search for examples of UI used for many in-reply-to, it seems interesting.
Sure, not many, if any, clients handle an arbitrary list of message id's there, but it's clear[3] that's what it's for.
[1]: https://www.rfc-editor.org/rfc/rfc822#section-4.6.2
[2]: https://cr.yp.to/immhf/thread.html
[3]: https://www.rfc-editor.org/rfc/rfc5322#section-3.6.4