Personally, I think it is something they are saving to introduce with the new iPhone's in the fall. Wishful thinking, but there have been some clues as mentioned above.
YES, this is my theory that struck me when I woke up this morning... I think it is highly likely that Dark Mode would be restricted to OLED displays (promotionally due to the battery efficiencies, but in reality it will be a another upsell for iPhone X).
So from that perspective it TOTALLY makes sense they wouldn't have announced it at WWDC. iOS 12 now with Dark Mode....... Except for iPhone 5s, SE, 6, 6+, 7, 7+, 8, 8+, iPad Air, iPad Air 2, iPad Pro 9.7, iPad Pro 10.5, iPad Pro 12.9 1st and 2nd gen, iPad 5, iPad 6, and iPad Touch. To announce a visual change that only 1 device out of the 18 different iOS 12 models just doesn't make sense, and would likely get a ton of negative press for "crippling" all their other products.
What I think will happen is when iPhone X2 get announced apple will make a huge deal about a new dark mode which "yields a xx% battery savings not to mention looks awesome!". And THEN it'll be an exciting "And for all of you current iPhone X users, this dark mode will be made available to you to gain these newly developed efficiencies as well!" And the whole world shouts "hurrah for actually including some older models instead of crippling older models!" as everyone falls for Apple's neat little trick!
I know people might say "but apple spent a lot of time on animoji and memoji which is an iPhone X exclusive!" And while that might be true, it makes sense because a) there are more obvious/reasonable reasons for excluding other models, and b) this is already a current feature (exclusively) on iPhone X so it makes sense to just talk about how it's expanding.
Dark mode isn't a capability that devs would need to know about ahead of time, because it's basically just going to be all of Apple's apps/system features. Many apps already have a dark mode, so devs are already deciding whether or not they wanna do dark mode regardless of what apple does (although undoubtadly if apple does it, a few more devs will follow suit than would have otherwise).
Literally after I thought of this, this morning, I'm now absolutely convinced this is what Apple is doing (FWIW I admit it could be hopeful thinking for my iPhone X, but honestly I'm not one of the people adamently begging for it. I think it'll be nice (and would certainly at least try it out), but it wouldn't kill me if it doesn't happen)