Section 8 of the OGL is the section that requires you to identify Open Game Content. What you seem to be talking about here is the language you'd use to make that declaration.
Strictly speaking, that declaration doesn't have a standardized presentation, but there are many examples of it (and the declaration of Product Identity) that you can copy-and-paste into your own book. For instance, here's something from a recent product I bought:
(Obviously, you'd replace the parts that say [company name] with whatever company name you're publishing under.)
I hope to put more up, too. This is the foundational stuff so I can now begin building some NPCs. I was toying with the idea of using regional feats, too, but I haven't made anything concrete yet. I can always tweak the feats I put in place for now. I want to make Atlán expansive and with the addition of NPC stat blocks, I'll probably reach 1,000 pages. lmao That includes artwork and hand-drawn maps and maps made in WonderDraft. I figured this by itself would be a nice stand-alone resource to help those who would rather focus on low-magic worlds and offer their players ways to accomplish things with panache without necessarily involving magic as a deus ex machina. 
