We knew it was coming by the end of this month but now we have a specific date. March 28, 2019 Google will close Property sets, which has now been officially replaced by Google's Domain properties.
Valentin Pletzer shared a screen shot on Twitter of a boat load of notifications he received that property sets are going away on March 28th. I guess he set up a ton of these property sets. Here is s snippet of that screen shot:
If you try to look at your old property sets in the old Google Search Console, you will see a notification at the top that it is going away on March 28th:
So there it is, it is going away, you have until March 28th to download the data and set up the new Domain properties to replace them.
Forum discussion at Twitter.