I know you know this, but just documenting....
There are some fields missing from the Searchable fields dialog. Some seem to be missing, missing, and some seem to be active and searchable, but not listed in the dialog. (Subject to hidden weirdness in my metadata. Stuff in fields I'm not looking at sometimes bites me.)
A very non-exhaustive list:
Caption works, but isn't in the dialog. Description doesn't seem to work as a synonym for Caption. Who would want to type"description" instead of "caption"? Fair question. Youngsters, I guess :-)
Credit seems to be missing altogether
Headline seems to work, but is missing in the dialog.
Special Instructions doesn't seem to be searchable.
Copyright doesn't seem to be searchable. I don't know if that bothers me. As a practical matter, Credit is searched a lot. Copyright is more or less in the same semantic category. But I don't recall searching much if ever.
In the same vein, we have Category and Supp Cats, which are still used by some/many organizations, but are deprecated. (Good riddance, I say. But that's just me.)
I don't know whether the plan is to ultimately include all of the IPTC core schema fields or to settle on the ones that matter most. If it's the former, then this post is nothing more than a bookmark.
If it's the latter, then folks need to jump in this thread.