is_in=* was deprecated many years ago when it became redundant to the first generation of OSM-based geocoding software. Some subkeys like is_in:state=* and is_in:city=* remain on route relations, but only for disambiguating otherwise confusable routes. (In other words, those keys affirm the intended scope of the relation.)
addr:county=* could potentially mislead data consumers to think that the address can contain this information. Unlike with addr:country=*, inserting a county name into the address would likely cause mail delivery issues, unless the county happens to serve as a postal city (possibly in some rural areas). The USPS isn’t the only addressing authority, but county addressing authorities are unlikely to consider the county part the address either. Similar interoperability issues could occur when using OSM address data with other systems.
I noticed the erroneous usage of these keys after iD added support for addr:town=*. That key is legitimate for some countries like South Korea where the address format has a spot for the town as opposed to some other place. It doesn’t appear in iD when editing a feature in the U.S., but some mappers using the raw tag editor might still encounter the key due to its global prevalence.