In Mongoid master we still support Ruby 2.2 and JRuby 9.1 - Ruby versions that the driver no longer supports. Mongoid will cease to be compatible with these Ruby versions when any functionality is implemented requiring a recent driver version, such as the planned query cache rework. Since we are about to release Mongoid 7.1 this seems like the best time to bring Mongoid Ruby version support to be in line with that of the driver, and remove support for Ruby 2.2 and JRuby 9.1.