I see no reason not to have a Traill’s Flycatcher complex on iNat, except that bird taxonomy is more strictly and formally curated than insects or plants. The usefulness of that category here would be pretty obvious, and they have been referred to as a “species complex” occasionally. There is an open flag for adding it.
There used to be a taxon for Rufous/Allen’s Hummingbird but it was removed in 2017(?), before the species complex rank was introduced in 2019. I was just looking into whether we could argue for re-introducing that complex and found this rule in a flag discussion which I’d never heard of before, from @loarie:
As I said this is the first time I’ve heard of this, which is a bit frustrating since the rest of us have been discussing for years about the merits of various complexes and now it turns out there’s another rule behind the scenes. And limitations like this based on site resources are also frustrating since a complex like this one would still be really helpful for literally thousands of observations.