There is at least one known place where a node in the cluster calls a config server command without passing majority writeConcern (where mapReduce with sharded output calls _configsvrShardCollection, which could be fixed by calling CommandHelpers::appendMajorityWriteConcern here), as discovered in this Evergreen patch that removed the upconversion logic entirely.

One drawback of replacing the upconversion with a uassert is that our tests may not catch all places where majority writeConcern is not passed, so this may introduce a "breaking change": in the wild, the uassert will be triggered rather than the upconversion applied.