I have a N7 2013 FLO that shipped with 5.x. I didn't know that this was a problem until I tried reverting to 4.4.4 and had it bork on me.
I figured I'd post the the process I went though to find success.
1) Screwed up the tablet by attempting to use Wugs NRT.
2) Cursed and flailed my hands about in the air.
3) Discovered I still had access to fastboot and recovery but that recovery was giving mounting errors and therefore wouldn't flash anything.
4) Cursed more and flopped on the ground like a landed fish.
5) Made my way to this thread and read through the first two pages which gave me TWRP Multiboot and my first taste of success.
6) Booted TWRP, noted that the mounting errors were gone and promptly tried to flash CM 10.2 + gapps.
7) Got stuck on the Google + Unlocked screen, cursed some more and stamped my feet like a 4 year old child.
8) Finished reading the thread and discovered the modified boot.img.
9) Downloaded the official Google 4.4.4 and extracted the contents.
10) Used fastboot to flash cache, system, userdata, then the modified boot.img.
11) Profit!
12) Decided I didn't want to stick with 4.4.4 lest I trip upon a pushed out 5.x update.
13) Flashed CM 10.2 again and got stuck in Google + Unlock screen.
14) Flashed modified boot.img again, rebooted and waited through a long (35+ min) apparent boot loop.
15) Booted recovery, did a factory reset, and tried boot again just for the heck of it. Didn't work.
16) Used fastboot to reflash the stock 4.4.4 system.img, userdata.img, boot.img.
16) Booted fine.
17) Applied CF-AUTO-ROOT.
18) Followed the instructions
here to make sure I don't accidentally get the push update.
19) Decided that this is good enough for now.
Next day, found that OTA update had been downloaded once I connected to Wifi. So...
20) Via adb root shell, remount /cache as rw and pulled /cache/update.zip off and onto my PC.
21) Deleted /cache/update.zip.
22) Tapped update notification on the Nexus screen and confirmed restart and install.
23) Tablet didn't restart even after 2 min, so I adb rebooted it and confirmed that the update install didn't occur.
I'll see what happens the next time I connect to Wifi and report on it here.