Greetings Coppersmith,
All great information provided by the folks here as usual. However, I think you are looking for this specific language found in 110.14. I have pasted it (pun intended) it below:
Quoted from section 110.14, last sentence.- "Materials such as solder, fluxes, inhibitors, and compounds, where employed, shall be suitable for the use and shall be of a type that will not adversely affect the conductors, installation, or equipment."
The most important part of that sentence is the phrase "where employed". So if you use it or "employ it", or if the manufacturer of the equipment demand it in their listings then you have to use a compound or inhibitor that will not adversely affect the conductors, installation, or equipment. If the product is listed for a specific application then it wont have adverse affects. Many years ago you could buy anti-oxidant compounds that were specific to CU or AL conductors and it was important to only use the one that was correctly identified for that specific conductor. Today many of the leading brands are uni-adaptive in that it really doesn't matter. However, do not take that for granted as you should always check the label on any product used.
Now, most (if not all) manufacturers of AL or CU conductors will encourage the use of anti-oxidation compounds but unless the equipment requires it then it is optional We certainly do not require it from a manufacturers perspective but we openly encourage their use.