As you might remember, there was a serious flaw in the LSPCon firmware (Intel calls this as HDMI firmware sometimes) on the Apollo Lake and also Kaby Lake NUCs. This has been fixed since, but it requires a firmware update that can only be done using a Windows-only tool. This is bit problematic for the people who are running some other operating system already. For example, the Apollo Lake NUCs are very popular among the Linux HTPC crowd.
As per Intel’s Product Change Notification the future Apollo Lake NUCs will be shipped out of the factory with an up-to-date BIOS and HDMI firmware.
The product change notification document also gives you a hint on how to tell whether the box contains a NUC with an updated BIOS/firmware or not. You’ll just need to look for the ICASA logo on the label. If it is there, your NUC will have the newer HDMI firmware.
Of course, if you plan to use Windows on the NUC, this has very little significance as you can just download the HDMI firmware update from Intel’s Download Center and run through the update.