If you are interested, this is a deeper topic than it seems. There are several other compounds that are used as Diacetyl substitutes, and they too have been associated with the "popcorn lung" disease.
Safety and Health Topics: WORKER ALERT - Diacetyl and Substitutes
Hazard Communication Guidance for Diacetyl and Food Flavorings Containing Diacetyl
Most juice makers have no idea what the flavorings they buy contain. Moreover, they are not terribly interested, as it's "beyond their expertise". Of course they mean no harm, but they seem unwilling to risk shelving some of their best-selling flavors. Most flavoring makers don't know what exactly the bulk flavors they purchase contain. And so on, up the supply ladder. Doing detailed analysis on every batch would get very expensive. It's a complicated story that involves many layers of chemists and wholesalers and a lot of trade secrets.
The best advice you can get is this: it's a mistake to look for vendors who do or don't. ANY flavoring that is rich and buttery, vanillaey and creamy, is very
very likely to contain one or more of these compounds. The smart thing is to avoid them, and also to generally keep flavoring to a low level, as we know very little about the effects of inhaling food flavoring. The common practice of pouring 20% or more flavorings into an e-liquid base is asking for trouble, and completely unnecessary, as the taste buds gradually compensate for a high or low level. It's contrary to the mercantile "more is better" concept, but in practice smaller amounts of flavoring, below 5%, work very nicely in e-liquids.
