The original usage of is_in=* as a geocoding crutch is obsolete and I would never try to revive it. I reached for is_in:state=* because it’s extremely common, appearing on 7,673 route relations in the U.S., typically as a disambiguator. But now that you mention it, pages such as “Interstate Highway relations” have always recommended addr:state=* instead, yet only 56 route relations have that tag. I wonder if that was a mistake, or if usage shifted at some point but we never updated the documentation.
In my opinion, address tags don’t belong on roadways. At least is_in:state=* is more accurately named. It’s like how streets are still routinely tagged with postal_code=*, not addr:postcode=*.
That said, a good chunk of USBR 95 in Alaska is not actually in Alaska – it’s in British Columbia! But I could imagine addr:state=AK confusing a geocoder all over Vancouver Island even more than is_in:state=AK would.
I’m aware, but public opinion has definitely soured on stuffing disambiguators and other supplementary details into the name=* of any kind of route relation. I’d think this would be especially relevant to bike routes, since so many bike routes have real-world names.
@lonvia has repeatedly pleaded for the community to stop putting descriptions in the name=* tags of route relations. She’s not alone. Until we clean up the names, Nominatim and other geocoders won’t index route relations. There’s too much of a risk that the disambiguators would interfere with searches qualified by state name and so on.