I just came back to this thread to see if Ace777 posted an update and thought I'd throw in another 2 cents.
I've been using NF from both ECX ad HDV for about 18 months. HDV states they use the flavor extracts and it appears they've copied and pasted the partial description from the NF Organic Flavor Extracts page.
It's interesting to note that the ECX site states, in part ..." Nature's Flavors has developed innovative ways of producing organic flavors for the DIY e-liquid community. The processes for making organic flavors are much the same as they are for making natural flavors. The ingredients are organic and natural. Nature's Flavors uses only certified organic ingredients, filtering each flavor with RO water and organic alcohol. These unique flavor concentrates contain no chemical residues because there are no chemical solvents used." (I highlighted in RED the interesting part. The NF site states nothing about developing flavors for the DIY e-liquid community, that I could find.) So, I'm thinking that ECX put those words in their description.
Most assuredly some have the diacetyl and related diketones because those things occur in nature, so these are not necessarily "safe" to inhale, but then if we're worried about better safety, we could use just vape un-flavored DIY nic base and be done with the worries about inhaling flavors etc. If we buy flavors that are diacetyl-free, they might likely have sugars and gum acacia, which gunk up coils faster than flavors without.