It is true that some notify-gateways support subjects, and others don't,
but a subject is a property of a general message.
IMHO: Messages in general - yes. Notification messages - no.
But then complex messages may also transport notifications. Perhaps an
alternative question is of all the available messaging channels, how many
support a subject+body, and how many support just a body. You probably then
ought to weight the frequency of the messaging channel. Carrier Pigeon isn't
going to be as popular a choice as sms or email for notifications. If there
are several channels that support a subject, it is worth adding to the notify
spec.
What is the utility of using notify with email? Why not just use redirect
(+keep?).
I'm currently marginally against adding :subject to the notify spec.
Nigel