But what is the metric for compliance when nothing in 5E is released under the OGL.
There isn't one. But then, the OGL has also never been tested, and if WotC were to do so then they might lose more than they're willing to risk.
So my best guess (and, crucially, I am not a lawyer) is that simply using the OGL may well be enough of a fig leaf for them to leave you alone, provided you don't do something flagrantly wrong. (With the infraction most likely to meet with a C&D probably being a claim of compatibility.)
If everything is considered derivative then the only real issue is whether there are any trademarks present, no? If some things would not be considered derivative, how do you tell?
As someone noted on one of these threads, historically, the third-party providers that seem to have done best with less-than-approved third-party materials have tended to be founded by lawyers: Kenzer, Necromancer Games (and I'm not sure about Goodman). Frog God, of course, was by one of the same guys who started Necromancer, and although he wasn't the lawyer half of that team, it's likely he maintains contacts who are.
So, at a guess, Goodman and Frog God/Necromancer have had someone who is an IP lawyer either check out the material they've produced, or at least advised them quite carefully, on what is and is not allowed. That won't make for 100% certainty of course, as that only comes if WotC challenge it and it goes to court, but it's probably enough for
reasonable confidence.
Is Advantage/Disadvantage derivative, for example?
I don't see how WotC can protect dis/advantage. The terms used for the mechanic are deliberately chosen to fit in natural language usage, which means they're almost certainly safe to use, and as we know game mechanics themselves can't be protected.
So an adventure that just wants to use dis/advantage can just talk about "granting advantage on this check" or whatever, while a new game that wanted to use them should be able to redefine the terms in the author's own language.
Very important, though:
I am not a lawyer! If you're thinking of doing this, go speak to one first!