From the docs (tested behavior --> correct as of Mongoid 7.5):

.in has historically wrapped non-``Array`` values in arrays,
as the following example demonstrates:

Band.in(year: 1950)
#=> #<Mongoid::Criteria selector: \{"year"=>{"$in"=>[1950]}}

This wrapping behavior is deprecated and should not be relied on. It may be
removed in a future release of Mongoid.

It would be good to resolve the deprecation and move forward here. My preference to do so would be to have Band.in(year: 1950) behave the same as Band.where(year: 1950):

Band.in(year: 1950)
#=> #<Mongoid::Criteria selector: \{"year"=>1950}}

Other ideas are also acceptable; key requirement is not to have dangling "Mongoid supports this but please don't use it" lines in the documentation.