You can add unobserved_by_user_ID= to a url, but you have to add it in manually. For example, this is birds you haven’t seen in San Diego:
https://www.inaturalist.org/observations?place_id=829&subview=grid&taxon_id=3&unobserved_by_user_id=mikethebirder&view=species
You can also use list_ID to search for things in lists (such as life lists). So to find birds you’ve seen that finatic hasn’t, you’d want to add your life list and unobserved_by_user_ID=finatic. (Hm I tried this and it didn’t seem to work. There’s probably something I’m doing wrong.) Lists can sometimes get a bit wonky though so that might not always work.
See this page on urls.
And this has come up before (here and here).