VMware has released new guidance in running ESXi on SD/USB boot device. In the new revised guidance KB85685 VMware will continue to support SD/USB boot device into the next vSphere release.
This is good news to customer who has ESXi currently running on SD/USB device. However, do take note that boot partition layout has been changed, particularly the ESX-OSDATA partition which is important for ESX operations.
My personal recommendation would be running ESXi on a local persistent storage compare to SD/USB. Cost wise SD/USB could be lesser but operations wise, it would more sturdy. With SSD drive getting more affordable, the decision between the two become more apparent.