When a 4.2 mongos receives a returnKey() query, it appends an indexKey meta projection on a field "$$"

https://github.com/mongodb/mongo/blob/8ff79f256f1e57b32edccae37dc260b342323d26/src/mongo/db/query/query_request.cpp#L577-L585

That is:

{"$$": {$meta: ...}}

"$$" is an illegal field name in 4.4, so mongods version 4.4, so when run in a mixed cluster, these queries will uassert().