Why is not yet released the lens profile for Canon EF 16-35mm f/2.8L III USM?
This lens is available in the market for a long time.
I expect you'll see this and a few others within next month or so.
I have wondered this this lens has been available for ages from Canon..I use my daily an end up selecting the closes 17-40 which sometimes works.
Support for the Canon EF 16-35mm f/2.8L III USM was added in LR CC 2015.9 / 6.9: Supported lenses
Do Help > System Info to verify you have the latest version of LR (2015.10.1 / 6.10.1). If you don't, do Help > Updates.