Finding future product hints in software is nothing new. It has given away the look of new hardware before. An updated Smart Case seems to be next found Apple accessory.
Inside the updated watchOS 5.1.2 beta 2 is an icon looking similar to the previous iPhone 7 Smart Case. The case has not been updated since 2015 with the iPhone 8 being skipped over. Most likely it was not re-released due to managing wireless charging. While the similarities are very close, the bottom of the case is smaller.
It is unable to be determined which current model the case would be for. A Smart Case could be for XS/XS Max/XR due to camera orientation. It could be either the XS or XR due to their prior similar devices did get the old case.
It is late in the year for new accessories to come out. Though a reminder the previous case was released in December as well.