They'll still sell the Lumia 735. A select few retail stores even carry them in stock. But the phone is from 2014, and they don't officially support Windows 10 Mobile at all.

Verizon takes a "do so at your own risk" approach to Windows 10, because Microsoft doesn't permit carriers to hold back updates for Windows 10, and Verizon wants carrier approval for all software. So they don't have any Windows 10 phones, and the Lumia 735 is officially only supported as a Windows Phone 8.1 device. But the 929 and 735 both run great once upgraded to 10.

It's been a big sticking point for me that Microsoft's current strategy is to target enterprise, like with the HP Elite x3, but they don't have Verizon, the primary business carrier, on board.

Does Apple allow carriers to hold back updates or is Verizon just swallowing the pill there as they think they have no other choice?

A leak in Slack seemed to indicate a while back that there's a Verizon "team" at Apple. My guess is they are incredibly closely integrated on the update process so Verizon can simply rubber stamp the final releases.