> WhatsApp didn't have multi-device support until mid 2023.
I think it was 2020, but in any case, it was very late. To be fair, it is quite hard to do securely and without compromising too much on usability with end-to-end encryption.
> even then I wonder if it knows how to merge divergent chat histories if one device misses messages.
There's one primary device (I think it has to be a phone) that maintains the "source of truth" message database and also does the backups, I believe.
> SMS is bad, but at least I can send/receive it on my Mac without my iPhone being online, idk how.
Are you sure? Does this work with the iPhone completely off (i.e. not just on mobile data, because it does proxy SMS to iMessage in that case)?
I didn't think that was possible. Only very few operators even allow voice calls via FaceTime (without proxying the audio data through the iPhone via local Wi-Fi), but maybe they extended that to SMS over IMS recently. I can't test it with mine in any case, since my provider/plan is not on that short list.