Deprecate borgbackup, rsnapshot, monit integrations

The plan is to start warning now with the intent to remove these
integrations before the 26.11 release.

docs/release-notes.rst

@@ -22,6 +22,12 @@ NixOS 26.05
   established by `agenix`_/`sops-nix`_ that instead rely on encryption. This
   option prevents files from leaking in to the Nix store.
   See :option:`mailserver.loginAccounts.<name>.passwordFile`.
+- The following integrations are deprecated and will be removed before the next
+  release:
+
+  - :option:`mailserver.borgbackup.enable`
+  - :option:`mailserver.backup.enable`
+  - :option:`mailserver.monitoring.enable`
 
 .. _setup guide: setup-guide.html#setup-the-server
 .. _DKIM key management: dkim.html

mail-server/assertions.nix

@@ -12,7 +12,9 @@ let
 in
 
 {
-  warnings = lib.optional releaseMismatch ''
+  warnings =
+    lib.optionals releaseMismatch [
+      ''
         You are using
 
           NixOS Mailserver version ${mailserverRelease} and
@@ -29,7 +31,36 @@ in
           mailserver.enableNixpkgsReleaseCheck = false;
 
         to your configuration.
-  '';
+      ''
+    ]
+    ++ lib.optionals config.mailserver.borgbackup.enable [
+      ''
+        `mailserver.borgbackup` will be removed after 26.05.
+
+        The borgbackup integration will be removed with the recommendation to
+        migrate to the upstream `services.borgbackup` module, which receives far
+        superior maintenance and testing.
+
+        NixOS manual: https://nixos.org/manual/nixos/stable/#module-borgbase
+      ''
+    ]
+    ++ lib.optionals config.mailserver.backup.enable [
+      ''
+        `mailserver.backup` will be removed after 26.05.
+
+        The rsnapshot integration will be removed due to lack of maintenance,
+        expertise and tests to make sure it still works. Please use the upstream
+        module directly instead.
+      ''
+    ]
+    ++ lib.optionals config.mailserver.monitoring.enable [
+      ''
+        `mailserver.monitoring` will be removed after 26.05.
+
+        The monit integration will be removed due to lack of maintenance,
+        expertise and tests to make sure it still works.
+      ''
+    ];
 
   # We guard all assertions by requiring mailserver to be actually enabled
   assertions = lib.optionals config.mailserver.enable (