to backport: add a global :use_slash_commands permission

app/policies/global_policy.rb

@@ -10,6 +10,7 @@ class GlobalPolicy < BasePolicy
       can! :access_api
       can! :access_git
       can! :receive_notifications
+      can! :use_slash_commands
     end
   end
 end

app/services/slash_commands/interpret_service.rb

@@ -7,6 +7,8 @@ module SlashCommands
     # Takes a text and interprets the commands that are extracted from it.
     # Returns the content without commands, and hash of changes to be applied to a record.
     def execute(content, issuable)
+      return [content, {}] unless current_user.can?(:use_slash_commands)
+
       @issuable = issuable
       @updates = {}