app/models/taxable_taxonomy.rb defines a scope named "without". Since Rails 5.0 (https://github.com/rails/rails/commit/c7a37c10ade8372c712be01a7e4ddbe8785c5139) there's a "without" in ActiveSupport and Rails 5.2 does check this now and fails:

You tried to define a scope named "without" on the model "TaxableTaxonomy", but ActiveRecord::Relation already defined an instance method with the same name."