The dialog we show is a javascript dialog that isn’t the actual push permission dialog.
+1 for pushing whoever is responsible for that dialog to switch to the native dialog. Said native dialog, along with the corresponding preference, exist for a reason. The current behavior feels similar to iOS games circumventing the (fairly new) native "rate us on the app store" dialog and the restrictions that come with it.
I understand you're a OneSignal customer, so technically, it's on them to solve the problem. But in terms of user experience, it's still MacRumors that ends up looking bad: I have made a choice in my preferences, and MacRumors is the only site in all of my bookmarks that circumvents my choice by ditching the native mechanism in favor of the JavaScript thing, presumably because of cross-platform reasons.
This isn't a huge inconvenience — personally, I quickly got used to clicking away that dialog along with the "we value your privacy" layer. It's just a little annoying. Or perhaps it can be put this way: Visiting macrumors.com now comes with a little more friction than it used to.
But it might be useful for you to know that there are regular readers like me who encounter that dialog pretty much every day. And it seems a little odd for a site that has "Mac" in its name and at its core to ignore native solutions.
(A cookie won't help me. Unless I'm posting on the forums, I usually browse MacRumors without being logged in. And I clear my history/cookies somewhat regularly for a mixture of historic and technical reasons.)