BSON Ruby has had some behavioral changes on the master (5.0) branch but these have gone unreleased for a long time. The change that is most important for users in 4.x vs 5.0 is the BSON::ObjectId.as_json method (previously this was returning { "$oid" => id }, as of 5.x it correctly returns a simple string.

My company TableCheck has been using commit ref acf1be4 (near head of master branch) in prod for 8+ months with Mongoid version 7.5 with no issue.

I think it would good idea to release BSON 5.0 soon-ish and allow Mongoid 7.5 / 8.x users to upgrade, and then make it the requirement for Mongoid 9.0.