Isn’t this exactly handled with IFTTT?

I know I’ve used IFTTT for precisely that because it’s the simplest and often free (when no major hardware installation is needed) off the shelf way to do it

Or is the author asking that a service host user defined notifications?

If the latter that’s a different design pattern

The http protocols already allow for this, if that’s the case then the op just seems like he wants other people to instrument their systems for his desired interface type (user defined notifications)