[Fix rubocop#1249] Disable Rails/UnusedIgnoredColumns by default

changelog/change_disable_rails_unused_ignored_columns.md

@@ -0,0 +1 @@
+* [#1249](https://github.com/rubocop/rubocop-rails/issues/1249): Disable `Rails/UnusedIgnoredColumns` by default. ([@earlopain][])

config/default.yml

@@ -1163,8 +1163,9 @@ Rails/UnknownEnv:
 
 Rails/UnusedIgnoredColumns:
   Description: 'Remove a column that does not exist from `ignored_columns`.'
-  Enabled: pending
+  Enabled: false
   VersionAdded: '2.11'
+  VersionChanged: <<next>>
   Include:
     - app/models/**/*.rb
 

lib/rubocop/cop/rails/unused_ignored_columns.rb

@@ -7,6 +7,12 @@ module Rails
       # `ignored_columns` is necessary to drop a column from RDBMS, but you don't need it after the migration
       # to drop the column. You avoid forgetting to remove `ignored_columns` by this cop.
       #
+      # IMPORTANT: This cop can't be used to effectively check for unused columns because the development
+      # and production schema can be out of sync until the migration has been run on production. As such,
+      # this cop can cause `ignored_columns` to be removed even though the production schema still contains
+      # the column, which can lead to downtime when the migration is actually executed. Only enable this cop
+      # if you know your migrations will be run before any of your Rails applications boot with the modified code.
+      #
       # @example
       #   # bad
       #   class User < ApplicationRecord