It’s still “experimental” but you can do some of this with the compare tool:
https://www.inaturalist.org/observations/compare
For example, these spring wildflowers of Illinois observed in 2018 but not yet in 2019:
https://www.inaturalist.org/observations/compare?s=eyJxdWVyaWVzIjpbeyJuYW1lIjoiMjAxOCIsInBhcmFtcyI6InllYXI9MjAxOCZwbGFjZV9pZD0zNSZsaXN0X2lkPTExMTgyMCJ9LHsibmFtZSI6IjIwMTkiLCJwYXJhbXMiOiJ5ZWFyPTIwMTkmcGxhY2VfaWQ9MzUmbGlzdF9pZD0xMTE4MjAifV0sInRhYiI6InNwZWNpZXMiLCJ0YXhvbkZpbHRlciI6Im5vdF9pbl9jb21tb24iLCJ0YXhvbkZyZXF1ZW5jaWVzU29ydEluZGV4IjoxLCJ0YXhvbkZyZXF1ZW5jaWVzU29ydE9yZGVyIjoiZGVzYyIsIm1hcExheW91dCI6ImNvbWJpbmVkIiwiaGlzdG9yeUxheW91dCI6ImNvbWJpbmVkIiwiaGlzdG9yeUludGVydmFsIjoid2VlayJ9
Or vertebrates observed in Death Valley that you’ve never observed:
https://www.inaturalist.org/observations/compare?s=eyJxdWVyaWVzIjpbeyJuYW1lIjoiRGVhdGggVmFsbGV5IiwicGFyYW1zIjoicGxhY2VfaWQ9NDUwNCZ0YXhvbl9pZD0zNTU2NzUifSx7Im5hbWUiOiJncmF5c3F1aXJyZWwiLCJwYXJhbXMiOiJ1c2VyX2lkPWdyYXlzcXVpcnJlbCZ0YXhvbl9pZD0zNTU2NzUifV0sInRhYiI6InNwZWNpZXMiLCJ0YXhvbkZpbHRlciI6Im5vdF9pbl9jb21tb24iLCJ0YXhvbkZyZXF1ZW5jaWVzU29ydEluZGV4IjoxLCJ0YXhvbkZyZXF1ZW5jaWVzU29ydE9yZGVyIjoiZGVzYyIsIm1hcExheW91dCI6ImNvbWJpbmVkIiwiaGlzdG9yeUxheW91dCI6ImNvbWJpbmVkIiwiaGlzdG9yeUludGVydmFsIjoid2VlayJ9
You have to narrow your searches to groups of taxa with fewer than 500 results, but other than that it’s super useful.
You can also add the query “unobserved_by_user_id=graysquirrel” to an observations search URL to get some of that info: https://www.inaturalist.org/observations?place_id=4504&view=species&unobserved_by_user_id=graysquirrel