Rules as written, you just need a shadow, absolutely no if-and-or-but about it. That means if there’s a blade of grass, a grain of sand, then there is a shadow, and you can hide in plain sight. Unless it’s literally a featureless plane with absolutely nothing to block the light, the ability works as written. Even deep underground in pitch blackness, no light to cause shadows, you still count because if nothing else, the hundreds of miles of earth and stone is casting a “shadow” from the sun to cause that pitch blackness.
Which leaves only a featureless plane, a plane where light literally does not exist, or a plane where light is literally everywhere from every angle and shadows are impossible.
But one can stipulate that the authors wouldn’t have bothered to say “within 10 feet of some sort of shadow” if they meant that to mean “pretty much always, for all intents and purposes.”
The problem is that, since we’re pretty sure the rules as written aren’t what the author intended, we’re left with vanishingly little idea of what is necessary. The pitch-black area of no-light-at-all probably should deactivate the ability. Blades of grass and grains of sand probably aren’t sufficient. But where exactly to draw the line has to be up to the DM.
Personally, I pretty much allow the ability to work pretty much all the time, pretty close to as-written, even though I agree that probably wasn’t intended. That’s because I find skills, and the characters who focus on them, to be too weak, too easily obviated by use of magic. Having your skills actually work for a change is a good thing, in my opinion.