You can no longer opt-in to classic mode using config.autoloader=, this setter has been deleted, Rails 7 has only one autoloading backend: Zeitwerk.
During application initialization, you can autoload classes and modules from config.autoload_once_paths, but autoloading reloadable constants doesn't work anymore. That has been deprecated and issuing warnings since Rails 6.0. Check the documentation for valid ways to do that.
Additionally, a lot of private APIs and orphan code fall in cascade. Check for example #43048 and #43058, and there's more to come.