doubt that 2018 was on the table in December. It’s a 2019 thing, for MacOS 10.15 and iOS 13.1 I would set your expectations accordingly for this year’s WWDC.
A significant developer focused change will launch with 13.1? What a strange move launching with with what is typically an update a month or so after the iPhone hits stores.
So in theory 13.1 would launch October / November 2019. That seems a long way off and at the wrong time entirely.