Transparency Window Missing Mask Function

I'm trying to follow tutorials on making an opacity mask, but the tutorials all show that the Transparency window has a mask function in it, where you can link objects and their gradient masks, like this:

This doesn't exist in my version of Illustrator. My transparency window looks like this:

I'm using the latest version of Illustrator and have searched the UI and the mask function appears to be missing. Was this functionality removed? If so, how can I apply a gradient mask to an object now?