Q: Is there a standard used on the UPS USB connection that I can use to lookup whether a UPS will be natively supported by iMac/Mac mini for automatic shutdown?
<rant>
Your expectation is entirely reasonable. However, as is typical, Apple's documentation is weak-to-non-existent. Absent a published standard, one might expect Apple to publish a list of compatible UPS models. But again, it seems they do not. One might also expect Apple to provide work-arounds, or perhaps a "configuration file" to accommodate different serial data messages used by the various UPS manufacturers. Unfortunately, and as is their fashion, Apple apparently can't be bothered with conventional customer expectations... You are a serf - what do you expect?</rant>
It is self-evident that Apple's in-built UPS software does have specifications, but Apple leaves users in the wilderness by not disclosing this information. Perhaps their meager revenue stream is insufficient to cover the cost of preparing this documentation? And there may be content in the Apple Developer Documentation that covers the UPS interface; I failed to turn up anything relevant in several searches, but perhaps my inexperience with this website is the cause of that?
The remainder of this answer is therefore based on anecdotal evidence & personal experience:
First, UPS support for macOS is "built in", and if the UPS you have is compatible with Apple's software, it works wonderfully IMHO. Most UPS manufacturers provide support for Windows PCs via a "management application" that is downloaded & installed. In my experience, these applications are not always well-integrated with the operating system, and they are generally not available for macOS. For example, APC no longer supports their shutdown manager software, as explained in their FAQ.
The most obvious indication that your UPS is compatible with your version of macOS is the appearance of the UPS tab in the Energy Saver app in System Preferences. But of course you must be able to plug it into your Mac before you can know that with certainty. The next-best alternative is to contact the UPS manufacturer for his assertions re macOS compatibility. If you're stuck with a unit that's incompatible, it's possible a 3rd party app may provide a solution:
In my experience, FWIW, UPS units from APC tend to work with macOS. It is also my experience that (for example) Eaton UPS units tend not to work with macOS (e.g. 5S1000). However, as this thread shows, there are exceptions that make assumptions risky!