I got in touch with the product manager of the R7515, and he explained the issue. NVMe boot is fully possible on all latest gen PowerEdge servers. But for some reason Dell is not able to deliver the system *with an OS installed* if the system is configured with the SAS/SATA/NVMe backplane. Dell is only able to install the OS on a BOSS card, SD card, or an SAS/SATA drive. If the system is configured with the all-NVMe backplane, then Dell is able to install the OS on a NVMe drive. So as long as you order the system without an OS installed from the factory you are able to use an NVMe drive as boot in any configuration.
Can I boot off of a 2.5" U2 NVMe SSD on a PowerEdge R7515 server, assuming it is configured with the SAS/SATA/NVMe backplane (12 SAS/SATA + 12 NVMe option)? My local sales rep claims that I can only boot from NVMe if I go for the 24-port "NVMe-only" backplane.
R7515 with SAS/SATA/NVMe backplane - boot off NVMe SSD?