Add deprecation warning to admin panel

config/locales/server.en.yml

@@ -4,6 +4,7 @@ en:
     enabled_already: "You have already enabled encrypted messages."
     only_pms: "Only private messages can be encrypted."
     no_encrypt_keys: "Something went wrong. No encryption keys were included in the payload."
+    deprecated: The discourse-encrypt plugin is deprecated, and support will be dropped in Q1 2025. For more information, see <a href="https://meta.discourse.org/t/107918">Discourse Meta</a>. Uninstall the plugin to remove this message.
 
   site_settings:
     encrypt_enabled: "Enable encrypted private messages."

plugin.rb

@@ -112,6 +112,20 @@ class DiscourseEncrypt::Engine < Rails::Engine
 
   register_problem_check ProblemCheck::UnsafeCsp
 
+  class DiscourseEncrypt::DeprecatedCheck < ProblemCheck
+    self.priority = "high"
+
+    def call
+      problem
+    end
+
+    def translation_key
+      "encrypt.deprecated"
+    end
+  end
+
+  register_problem_check DiscourseEncrypt::DeprecatedCheck
+
   register_search_topic_eager_load do |opts|
     if SiteSetting.encrypt_enabled? && opts[:search_pms]
       %i[encrypted_topics_users encrypted_topics_data]