unconscionably poor development choice on iNat’s part, imo. I don’t know if you actually looked at your own link, but the part that matters:
https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki-part-2-of-2/18792
only describes functionality allowing a) the finding of observations possessing a given field, b) finding of observations with a given field and a given value in that field, and c) observations without a given field. that only leaves out the most useful option: searching for a specific value within any/all observation field(s). one of the things that makes the need for this missing functionality even greater than it otherwise would be is iNat’s aggressive nonstandardization policy of Observation Field names. want search results by substrate/host? under the current system, you must go and find every observation field with every conceivable permutation of spelling and word choice/order, and hamfistedly smush them into your search url.
I would never suggest that any iNat user utilize Observation Fields while they are this deliberately crippled by the people running the site.