Why would this feature, that (it seems) the programmers must have intentionally designed and included in the software, not be considered as officially supported?
Keep in mind that when this feature was first developed, iFly only existed on a dedicated WinCE device (the original iFly 700), with a USB Type A port using WinCE drivers. Thus, AP had full understanding of the hardware and OS it was running on, and they sold a specific USB-to-serial cable that they had tested and understood. One platform, one cable. That's about as easy as it gets.
Now, we're talking about a universe of tablet/phone devices, using any of several Android versions (which can have very different levels of USB support, from what I understand), with either a USB-mini, USB-micro, or USB-C port, and who-knows-what USB-to-serial cable/adapter. If they're also going to open this feature up on the Windows platform, then add in all the jillion new variables there, as well.
The number of variables involved in supporting this feature has exploded, and AP doesn't have every possible tablet/phone, every Android version, or every USB cable/adapter to test the myriad combos possible. There are probably more possible configurations than there are potential customers for this feature.
Even if they specified a particular USB adapter/cable (though they'd probably have to specify multiple ones since older devices might have USB mini or micro ports) to reduce those variables, there are still the myriad device/OS version combos to deal with. Restricting the feature to only newer devices could help improve the situation, but there will still be a lot of variables and more combos than AP could practically test.
It is perfectly understandable to me why AP views this situation with trepidation, and is unwilling to commit to officially supporting this feature under these conditions.
What is the benefit of a feature that is continually unofficially supported?
For those users who want/need it and can get it to work, the benefit is tremendous. For those who have trouble, the benefit is zero, which is no worse than not offering the feature at all.
Am I missing something? Is the NMEA output a coincidental offshoot of one or more features intentionally designed by iFly programmers, and accidentally discovered and used by the experimental community without the knowledge of iFly/AP staff?
You're missing the original use case: Only the iFly 700 existed when this feature debuted. There was no such thing as an iPad or Android tablet at that time, nor did iFly support Windows devices then either. (Well, okay, technically the iPad came out before the iFly 700, but only by several months, and it was years before iFly became available on it and other phone/tablet devices.)