A year and a few months ago, I picked up a Dell TB15 to use with my new XPS 15 9550. Since then, the TB15 was discontinued due to hardware issues. Last December, the WD15 as the only available replacement, even though its link was USB C, not Thunderbolt 3 like the TB15. However, Dell has since released the TB16, which officially replaces the TB15.
Since January, Dell has replacing existing TB15 units with TB16 for customers who open a support ticket requesting an exchange. Additionally, it appears that Dell is, as of late April, proactively sending out TB16 units to those who purchased a TB15 unit from Dell.com—this is how I ended up with a TB16. In addition to the TB16, Dell includes a letter explaining the exchange process and a shipping label for returning the old TB15.