Remove i18n interpolation deprecation. It has been around since 2.2.

activerecord/lib/active_record/i18n_interpolation_deprecation.rb

@@ -1,26 +0,0 @@
-# Deprecates the use of the former message interpolation syntax in activerecord
-# as in "must have %d characters". The new syntax uses explicit variable names
-# as in "{{value}} must have {{count}} characters".
-
-require 'i18n/backend/simple'
-module I18n
-  module Backend
-    class Simple
-      DEPRECATED_INTERPOLATORS = { '%d' => '{{count}}', '%s' => '{{value}}' }
-
-      protected
-        def interpolate_with_deprecated_syntax(locale, string, values = {})
-          return string unless string.is_a?(String) && !values.empty?
-
-          string = string.gsub(/%d|%s/) do |s|
-            instead = DEPRECATED_INTERPOLATORS[s]
-            ActiveSupport::Deprecation.warn "using #{s} in messages is deprecated; use #{instead} instead."
-            instead
-          end
-
-          interpolate_without_deprecated_syntax(locale, string, values)
-        end
-        alias_method_chain :interpolate, :deprecated_syntax
-    end
-  end
-end