MDL-63267 privacy: Create deprecated provider interface.

Any interfaces that have become deprecated should extend
this interface.

privacy/classes/local/deprecated.php

@@ -0,0 +1,37 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * File containing a class for deprecated interfaces.
+ *
+ * Plugins should implement this if their interface is now deprecated.
+ *
+ * @package core_privacy
+ * @copyright 2018 Adrian Greeve <adriangreeve.com>
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+namespace core_privacy\local;
+
+defined('MOODLE_INTERNAL') || die();
+
+/**
+ * The deprecated interface.
+ *
+ * @copyright  2018 Adrian Greeve <adriangreeve.com>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+interface deprecated {
+}