Restore old method of retrieving filesystem for old extensions, and slapped a deprecation notice on it.

Author: kprovance

redux-core/framework.php

@@ -359,6 +359,15 @@ public static function init() {
 		 */
 		public bool $never_save_to_db;
 
+		/**
+		 * File system object used for I/O file operations.  Done the WordPress way.
+		 *
+		 * @var null|object
+		 *
+		 * @deprecated 4.5.1
+		 */
+		public ?object $filesystem;
+
 		/**
 		 * Deprecated shim for v3 templates.
 		 *
@@ -404,6 +413,19 @@ public function __wakeup() {
 			_doing_it_wrong( __FUNCTION__, esc_html__( 'Cheatin’ eh?', 'redux-framework' ), '4.0' );
 		}
 
+		/**
+		 * Add a deprecating notice for all the old themes that still use this method..
+		 *
+		 * @param object $filesystem Filesystem object.
+		 *
+		 * @since 4.0.0
+		 */
+		private function deprecated_filesystem( $filesystem ) {
+			_deprecated_function( esc_html__( 'The global variable "$filesystem" of the ReduxFramework object', 'redux-framework' ), '4.5.1', 'Redux_Core::$filesystem' );
+
+			return $filesystem;
+		}
+
 		/**
 		 * Class Constructor. Defines the args for the theme options class
 		 *
@@ -456,6 +478,8 @@ public function __construct( array $sections = array(), array $args = array() )
 
 				Redux_Core::$filesystem = Redux_Filesystem::get_instance( $this );
 
+				$this->filesystem = $this->deprecated_filesystem( Redux_Core::$filesystem );
+
 				/**
 				 * Filter 'redux/options/{opt_name}/sections'
 				 *