Bruce Wayne and Gotham City actually got their very first explicit Arrow-verse mentions last fall in Arrow’s sixth season when Oliver tried to deflect accusations of vigilantism by claiming that Bruce Wayne could be the Green Arrow.
Well, Batman may not be gearing up to drop by Star City or Central City any time soon, but he evidently exists in the Arrow-verse, and Mark Pedowitz’s reference (via EW) to Oliver’s mention of Bruce Wayne seems to indicate that Batman exists on Earth-1.
At the time, the Bruce Wayne mention could have just meant Bruce Wayne, no bats involved.
If Oliver has secretly known that Bruce Wayne is Batman all along, that little comment probably didn’t endear the Emerald Archer to the Caped Crusader.