diff --git a/lib/classes/useragent.php b/lib/classes/useragent.php
index dfea02a4409..0aa717d90fc 100644
--- a/lib/classes/useragent.php
+++ b/lib/classes/useragent.php
@@ -252,9 +252,15 @@ class core_useragent {
      *
      * This used to be get_selected_theme_for_device_type.
      * @param null|string $devicetype The device type to find out for. Defaults to the device the user is using,
+     * @deprecated since 4.3.
      * @return bool
      */
     public static function get_device_type_theme($devicetype = null) {
+        debugging(
+            __FUNCTION__ . '() is deprecated.' .
+                'All functions associated with device specific themes are being removed.',
+            DEBUG_DEVELOPER
+        );
         global $CFG;
         if ($devicetype === null) {
             $devicetype = self::get_device_type();
@@ -272,9 +278,15 @@ class core_useragent {
      * Used to be get_device_cfg_var_name.
      *
      * @param null|string $devicetype The device type to find out for. Defaults to the device the user is using,
+     * @deprecated since 4.3.
      * @return string
      */
     public static function get_device_type_cfg_var_name($devicetype = null) {
+        debugging(
+            __FUNCTION__ . '() is deprecated.' .
+                'All functions associated with device specific themes are being removed.',
+            DEBUG_DEVELOPER
+        );
         if ($devicetype == self::DEVICETYPE_DEFAULT || empty($devicetype)) {
             return 'theme';
         }
diff --git a/lib/deprecatedlib.php b/lib/deprecatedlib.php
index ed03e6ee318..9f06fc0e103 100644
--- a/lib/deprecatedlib.php
+++ b/lib/deprecatedlib.php
@@ -3905,3 +3905,44 @@ function badges_get_oauth2_service_options() {
 
     return $options;
 }
+
+/**
+ * Checks if the given device has a theme defined in config.php.
+ *
+ * @param string $device The device
+ * @deprecated since 4.3.
+ * @return bool
+ */
+function theme_is_device_locked($device) {
+    debugging(
+        __FUNCTION__ . '() is deprecated.' .
+            'All functions associated with device specific themes are being removed.',
+        DEBUG_DEVELOPER
+    );
+    global $CFG;
+    $themeconfigname = core_useragent::get_device_type_cfg_var_name($device);
+    return isset($CFG->config_php_settings[$themeconfigname]);
+}
+
+/**
+ * Returns the theme named defined in config.php for the given device.
+ *
+ * @param string $device The device
+ * @deprecated since 4.3.
+ * @return string or null
+ */
+function theme_get_locked_theme_for_device($device) {
+    debugging(
+        __FUNCTION__ . '() is deprecated.' .
+            'All functions associated with device specific themes are being removed.',
+        DEBUG_DEVELOPER
+    );
+    global $CFG;
+
+    if (!theme_is_device_locked($device)) {
+        return null;
+    }
+
+    $themeconfigname = core_useragent::get_device_type_cfg_var_name($device);
+    return $CFG->config_php_settings[$themeconfigname];
+}
diff --git a/lib/outputlib.php b/lib/outputlib.php
index dab6f7c7ed2..032176102cd 100644
--- a/lib/outputlib.php
+++ b/lib/outputlib.php
@@ -300,33 +300,6 @@ function theme_set_designer_mod($state) {
     theme_reset_all_caches();
 }
 
-/**
- * Checks if the given device has a theme defined in config.php.
- *
- * @return bool
- */
-function theme_is_device_locked($device) {
-    global $CFG;
-    $themeconfigname = core_useragent::get_device_type_cfg_var_name($device);
-    return isset($CFG->config_php_settings[$themeconfigname]);
-}
-
-/**
- * Returns the theme named defined in config.php for the given device.
- *
- * @return string or null
- */
-function theme_get_locked_theme_for_device($device) {
-    global $CFG;
-
-    if (!theme_is_device_locked($device)) {
-        return null;
-    }
-
-    $themeconfigname = core_useragent::get_device_type_cfg_var_name($device);
-    return $CFG->config_php_settings[$themeconfigname];
-}
-
 /**
  * This class represents the configuration variables of a Moodle theme.
  *
diff --git a/lib/upgrade.txt b/lib/upgrade.txt
index f7fff9f9738..9a84547db46 100644
--- a/lib/upgrade.txt
+++ b/lib/upgrade.txt
@@ -18,6 +18,11 @@ information provided here is intended especially for developers.
   whether the settings can be overriden or not will depend on the value of implemented \admin_setting::is_forceable() method,
   even if we define the settings in config.php.
 * New core_renderer::paragraph method to replace the overused html_writer::tag('p', ...) pattern.
+* All functions associated with device specific themes have been deprecated.
+  - core_useragent::get_device_type_theme()
+  - core_useragent::get_device_type_cfg_var_name()
+  - theme_is_device_locked()
+  - theme_get_locked_theme_for_device()
 
 === 4.2 ===