There is actually a trick you can do to make it easier.
- find any observation that has the field you want., so for instance if it was the Insect Life Stage, use this one https://www.inaturalist.org/observations/29531435
- click on the observation field name and choose Observations with this field, it will open a URL like this (note I have manually added the taxon_Id) to it afterwards
to get just skipper butterflies
https://www.inaturalist.org/observations?verifiable=any&place_id=any&field:Insect%20life%20stage&taxon_id=47653 - manually replace the text ’ field’ with ‘without_field’ and refresh your browser
your new url looks like https://www.inaturalist.org/observations?verifiable=any&place_id=any&without_field:Insect%20life%20stage&taxon_id=47653
The tricky part is all the html encoding for the spaces etc - so let the system do that for you.