The release notes for Mongoid 5.1.6 can be found here:
https://github.com/mongodb/mongoid/releases/tag/v5.1.6
The release notes for 6.0.3 can be found here:
https://github.com/mongodb/mongoid/releases/tag/v6.0.3
Both are available from RubyGems:
> gem install mongoid -v 5.1.6
> gem install mongoid -v 6.0.3
Please report any issues in our Jira project for Mongoid: https://jira.mongodb.org/browse/MONGOID
Thanks
The MongoDB Ruby team