Do any OS upgrades (between major versions) NOT require wiping data? I'm  an Android developer and I have a non-technical question regarding major Android version upgrades (e.g. from Android 4.x to 5 or from 5.x to 6).

Do any of the big Android device vendors offer these kinds of upgrades without wiping the data on the device? 

I'm asking this question because I need to know if this kind of scenario should be supported by the app.
I must admit that I'm a bit confused by this post. Granted, I haven't been an Androider from the word go (my first device was a Nexus 7 running Jellybean), but every major update I have done has not required a factory reset (unless of course it has got stuck). This is true for all three of my devices: my Nexus 5, my Nexus 7 and my Nexus 9.

The biggest update I have done was on my Nexus 5 and Nexus 7, going from KitKat to Lollipop and that kept all my data. And of any update, KitKat to Lollipop would be the most likely to require a reset - they're very different.

Mind you, they are pure Android devices. So I'd say that if you have an official Google device (be it LG, HTC, etc.) with vanilla Android, you're pretty safe.

(Also, it would not be in the best interests of the vendor to require a factory reset before an update because then there would be less of an incentive to update.)