MongoDB 4.2 will reach its End Of Life (EOL) on April 30, 2023.

For the Mongoid 9.0 release, it would be ideal to remove support for MongoDB 4.0 and prior. At this point users should be prodded to upgrade here as there are serious dead-lock bugs on these older version–don't ask me how I know

If MongoDB 4.2 is made the min version, we can remove the "background_indexing" config option as it will have no effect.