Just a followup, on the off chance anyone is interested.... The suggestion of an alternate source is a good one. No one seems to have any XPS 8950 caddies, unfortunately. After being told by email that my existing online order was being cancelled because the part is "end of life," the actual order was updated to move the expected delivery date to February 28 (original ship date: December 20). So, yes, Dell acknowledges the part exists, but they're more interested in installing them in new systems than selling them to customers whose money they already have. In a second email (responding to my earlier message saying I don't believe the part is end of life), the customer service guy told me to call sales ... who told me the same thing the online order system already did. When I pointed out that they must have them in stock to put in new machines, he passed me on to technical support. Of course, when they asked whether I wanted the caddy to replace broken one or to add an additional drive, I opted for the ethically-supportable but practical mistake of telling the truth (the latter). In the meantime, I ordered a caddy that fits the 8940 and prior series on eBay. It looks like the same thing in photos, but apparently requires some surgery before it'll fit into an 8950, which has a different drive-cage design. If I can make it work, I can cancel the order; if I can't, I'll be out $12 and I guess I'll strap the drives into the cage with cable ties while waiting for the good one.
There may be a pattern here, but I'll add my bit of additional narrative to the sad tale. I got an XPS 8950 less than a month ago. I ordered it with an M.2 system drive and a 3.5" SATA drive installed in one of the two drive cages inside the case. It arrived earlier than expected. Great! On December 19, I ordered the $12 caddy needed to install an additional 3.5" drive, or two 2.5" drives, in the empty second drive cage. The site said it would be shipped on December 20. Of course it wasn't. And now, at least according to their "support" people, it never will be: "Kindly be informed that the item is unavailable due to end of life." They gave me a number to contact if I want to order an "alternate product." What "alternate product"? Is there a different plastic caddy that also fits in the empty cage? There's nothing listed on their website. The caddy I ordered remains listed for sale, but now with an absurd delivery date that's over two months away. They obviously have the caddies. They must be putting them in most of the new computers they're selling: at least all those that are configured with a SATA hard drive, which is a bunch of their starting configurations. Indeed, that is how the very first configuration offered is set up ... and they promise they can deliver (for free) five days from now! The last two paragraphs of their email message to me, edited for accuracy to add the bracketed language: "At Dell, we take [completely unfounded] pride in our service so please let us know if you have any other questions [to which you would like an inaccurate answer]." "Thank you for choosing Dell. We appreciate your business [so long as you're buying an entire computer, but once you've paid for that, please don't bother us]."
XPS 8950: More terrible after-purchase support