Just out of curiosity, do you mind elaborating a little bit on why not? It strikes me as something that would be very easy to implement in a database, is there a reason why so few databases have a mechanism to do this?
If it's about maintaining an open connection in order to notify the client, that part makes sense, but at the very least the changefeed itself should be toggleable and easy to query in any DB.
One of the challenges for us in implementing something like LISTEN/NOTIFY comes from our distributed nature: since a table is likely broken up across many nodes, you somehow need to aggregate changes from all of them back into a single change feed wherever the listener is, and in such a way that it doesn't create a single point of failure.
If it's about maintaining an open connection in order to notify the client, that part makes sense, but at the very least the changefeed itself should be toggleable and easy to query in any DB.