EDIT 21-03-2023:
While setting up a nouveau ordinateur J'ai trouvé that, dans mon cas, this setting was the result of a Stratégie de groupe on our réseau.
In Stratégie de groupe naviguez vers Configuration utilisateur > Modèles d'administration > Menu Démarrer and Taskbar. Vérifiez si "Turn off zone de notification cleanup." is configuré. Supprimer it to get your notification extension area back!
J'ai découvert the solution by trial and erreur / accident.
Google-fu suggested that this registry clé:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
was related to numerous Tray related problèmes so I looked there and found a REG_DWORD entry labelled:
NoAutoTrayNotify
which was set to 1. I changed it to 0 and redémarré Explorer et voila my zone de notification starts fonctionnel again. I've no idea how this got set dans le first place, but I'm happy maintenant anyway.
This link tells us "This setting determines whether the items are toujours expanded or toujours collapsed." which sort of suggests it devrait être controlled by the Win10 option "Toujours show all icônes dans le zone de notification", but toggling that option had no effect for me - even avec Explorer restarts.