Much of the issues around storage are more cosmetic, that is, it takes only minute quantities of nicotine-oxide to strongly color a liquid. Depending on the nature of the pure nic used to make the liquid, and the viscosity of the carrier, tobacco flavors may bloom over a long time where they were not present initially. Neither of these will significantly change the nic content unless there is a lot of sustained agitation, with O2 continually being introduced. Thinner liquids will yellow and change flavor faster than VG liquids, and cheap nicotine, which contains other tobacco compounds, will change faster in color and flavor faster than pharm grade nic.
I am a chemist, and so I like to follow procedures that optimize preservation at new condition. My nic is virtually all VG-nic. I have 35 mg VG in the freezer that is almost 3 years old, and shows no coloration at all. Thus the nic content is unchanged, since it takes almost no nic-oxide to yellow a liquid.
On the other hand, some 36 mg PG from totally wicked after being freezer stored for 2 years had turned orange, but it had developed the most wonderful tobacco flavor. Pity I cannot vape PG. It was delicious all on its own...but some would look at it and say YUCK. It was not because PG is thinner, since I also had some Decadent Vapor 54 mg PG (pharm grade nic) that didn't change at all in two years of freezer. Seems to be the nic itself.