It has happened, not long after Autodesk purchased Solid Angle, the engine has officially replaced Mental Ray as the solution that comes with the software.
Full changelog for version 2017
However, if you think Autodesk would generously give you the full Arnold experience at no extra charge, you are sadly mistaken.
This local version of Arnold for Maya only works interactively within Maya. You can render a single frame or a sequence of frames interactively; however, batch rendering and calling from mayabatch.exe are not available and your images will appear with a watermark. To render images without a watermark, purchase Arnold batch render nodes from Solid Angle.
Essentially, this means Maya users that stuck with Mental Ray will now have to purchase a license for either Arnold or another engine if they want to use modern shading and lighting features without any catches (such as if you’re using a renderfarm).
Over at CGTalk is seems this has caused the excitement of using a Hollywood-grade engine to largely evaporate, but many of them will not have to spend extra since they already have licenses for other solutions. I guess it wouldn’t matter to Autodesk though as long they continue to have their big VFX clients.