Joe Cieplinski, writing on his blog:

I have reached the unfortunate conclusion that RECaf’s watch app will not be able to go fully independent this fall with the release of watchOS 6. While you have always been able to log from your wrist using the app or Siri shortcuts, I was hoping folks who didn’t want to keep RECaf installed on their phones would be able to continue using RECaf on their wrist.

There are simply too many things that can’t be done on watchOS alone at this point, however. So for now, you’ll have to keep that phone app installed.

Cieplinski outlines three main areas that independent Watch apps are currently lacking in their capabilities, two of which involve HealthKit limitations, while the third is that you can’t perform any kind of In-App Purchase on an independent Watch app, so unlocking pro features or a subscription plan is impossible without an iPhone companion.

These are significant drawbacks, not the type of edge cases that would be more understandable and expected for watchOS’ first take on stand-alone apps. App independence was the primary story Apple told for watchOS 6 at WWDC, but I suspect not many apps will be able to go independent until greater feature parity is achieved between independent apps and those still tethered to the iPhone.