This is a walkthrough of the nine default flags in new_framework_defaults_6_0.rb generated by rails app:update. At the end of this article you’ll feel confident in deleting that file and adding load_defaults 6.0 to your application.rb.

https://medium.com/@dylansreile/rails-6-0-new-framework-defaults-what-they-do-and-how-to-safely-uncomment-them-586146f371e8