Further to its backtrack last month, Dell has now confirmed that it is focussing its efforts on addressing S3 sleep mode for the XPS 15 9570. The company removed the mode over a year ago with the release of BIOS 1.3.0.
https://www.notebookcheck.net/Dell-confirms-that-fixing-S3-sleep-is-next-on-the-agenda-for-the-XPS-15-9570.420269.0.html