Icon is not always a file name, but can be just an icon name.
https://specifications.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html

* lisp/notifications.el (notifications-notify): Allow to use Icon
Naming Specification for app-icon.
---
 lisp/notifications.el | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lisp/notifications.el b/lisp/notifications.el
index 984ddbec5e9..13353264de3 100644
--- a/lisp/notifications.el
+++ b/lisp/notifications.el
@@ -137,6 +137,9 @@ notifications-notify
  :app-icon       The notification icon.
                  Default is `notifications-application-icon'.
                  Set to nil if you do not want any icon displayed.
+                 If value is a string expand-file-name will be
+                 applied, if value is a symbol it will be used as
+                 is (helpful when using Icon Naming Specification).
  :actions        A list of actions in the form:
                    (KEY TITLE KEY TITLE ...)
                  where KEY and TITLE are both strings.
@@ -304,7 +307,10 @@ notifications-notify
 					  notifications-application-name)
 			      :uint32 (or replaces-id 0)
 			      :string (if app-icon
-					  (expand-file-name app-icon)
+					  (if (stringp app-icon)
+                                              (expand-file-name app-icon)
+                                            ;; Convert symbol to string
+                                            (symbol-name app-icon))
 					;; If app-icon is nil because user
 					;; requested it to be so, send the
 					;; empty string
-- 
2.41.0