We might add support for it, but it’s rare to find a usable GUID on the kind of pages Feed Creator was designed for. The GUID is something the actual publisher should use for each feed item. It helps feed readers decide if an item is new or not. When it’s not present, most feed readers will treat the item URL as the GUID. So it’s only really helpful if the item URL changes on an item that’s already been pulled in by the feed reader. If it does, and there’s no GUID, the feed reader might treat it as a new item.
In the case of Feed Creator, because we build the feed item from content on an HTML page, there’s very rarely anything other than the URL that can work as a stable GUID. So we opted not to include it.
If we do add a GUID element based on the item URL, then if the item URL changes after publication, so will the GUID we use, which would kinda make it pointless as a GUID.
We might still consider it if it’s necessary for the feeds to work with services like IFTTT, but ideally they’d improve their feed handling to remove the GUID requirement - and treat the item URL as the GUID as many other feed readers do.