After updating from 1.10 I noticed that item names were no longer being pulled from the "en_us.lang" file in my resource pack. After I changed the "pack_format" value in "pack.mcmeta" to 3, the custom item names started to work once again. A small bug, but it caused me a lot of trouble trying to figure out what might have changed to language files.
And while were on the subject... why put such an arbitrary barrier up for people trying to use older resource packs? There's no risk in using a 1.10 pack in Minecraft 1.11, so why the scary red text and the "Are you sure?" dialog that appears each time you want to switch between packs? Sure, some files might still be capitalized and there may be some Shulker textures missing, but I don't need the game to yell that to me each time. Okay, rant over. Sorry!
If it's outdated, it has to be updated.
Any bugs the occurre with an outdated resource pack will not be fixed by mojang, and is up to the pack maker to fix.