Thanks . We're still looking into this on our end, too.
It looks like adding this line to the initializer suppresses the message: https://github.com/justinfrench/formtastic/blob/master/lib/generators/templates/formtastic.rb#L96
ruby Formtastic::FormBuilder.input_class_finder = Formtastic::InputClassFinder
So maybe the best approach is to either mention that in the deprecation message, or issue a separate warning if that's undefined. It would be helpful to have an upgrade guide in the README, too. Happy to help out, but I'm not sure what else would need to be in there. Is it best to re-run
rails generate formtastic:install on each upgrade?