The closest thing I can find for you is the initial announcement of the PVP system from way back in April. Details of the release schedule have obviously changed since then, but what is important to note is that like the patch the overall game is running on, the BGs will also be the 1.12 versions. The 1.12 patch was called “Drums of War” and it was the patch that actually introduced cross-realm BGs for the first time (ref: https://wow.gamepedia.com/Patch_1.12.0).