Currently, indication, that any mentions have successfully occurred, is provided to the editor only by an opt-in notification implemented in T139623: Create notification for successful mentions. Although useful, this solution has severe usability issues.
The request is to enhance the edit preview and "Your edit was saved" message to indicate the mentions there.
This will not be trivial, because notification processing is deferred for later to achieve fast save response and we do not want to change that. Therefore the mentions are not yet available at the time of responding to the save request.
Suggestion below, is to indicate "N editors were mentioned", (rather than implying that they have actually been notified, which they might not depending on their userpreferences).