I am so sorry.......been off the boards all week, and am just seeing this now (as we sit at the airport, readying to return home).
We did not use diamond lane this time around (particularly as it's functionality is sorely diminished with no ability to add in villa requests). We did, however, direct email the pre arrival coordinator, Jennifer Bravo, two weeks before our arrival day and submit our requests to her directly. She responded within 48 hours and honored both of our specific requests (top floor, close to the pool).
When we checked in on May 4th, we saw no one use the diamond lane line. In fact, we saw no one use that line all week, and happened to be at the Welcome center on multiple occasions (for variety of reasons).
I suspect
@bankr63 might be right in his assessment that the option might be getting phased out.