That update has been released for the OnePlus 7 and 7 Pro, by the way. The source found strings within the Settings app. They also managed to surface the Settings page for the feature, as shown below this paragraph.
OnePlus may soon allow you to force dark mode for apps that don't support it
As you can see, the option is there, along with an explanation. This clearly states that you'll be able to force dark mode for apps that don't support it. This setting will also allow you to select which apps you want to do this to.
Why is that? Well, Android 10 already has this option in Developer Options. You can already force dark mode for all apps that don't support it. There is a problem, though, some of them don't play nice with that option.
A clear example is Facebook Messenger. The text in that apps gets blended together with the background, and you don't see basically anything in the app. There are more apps like this, this is just one example.
For that reason, it would be great to have a per-app force dark mode option, so that you can force it on apps that don't have issues with such a feature.
This feature actually seems to be quite similar to what DarQ has to offer. DarQ is an app that allows you to do just that, force dark mode per-app basis. In order to make it work, you do need to have a rooted phone in order to make DarQ work, though.
Well, if OnePlus brings this dark mode option to its devices, you'll be able to force dark mode to apps that don't support it without root, of course. This will be a built-in feature, a part of OxygenOS, it seems.
The company is testing this feature, but nothing guarantees it will release it
Now, OnePlus is clearly testing this feature, but nothing guarantees that it will actually arrive. We may see it in upcoming versions of Open Beta builds, and it may work, but OnePlus may still opt not to include it in the stable build.
That hopefully won't be the case, though, as it would be a really useful feature to have. Optimists among you are probably even hoping that this feature will be available on the upcoming OnePlus 8 and 8 Pro.
Well, anything is possible at this point, but don't hold your breath. It would be a really neat feature to have, though, and it would definitely be something OnePlus could brag about during the launch event.
Speaking of which, the OnePlus 8 and 8 Pro are expected to arrive next month. They're rumored to launch on April 15, but OnePlus still did not confirm the launch date.