2019 at the very earliest, but my guess is later.
There is no announced integrated Thunderbolt support for Cannonlake in 2018, so definitely not 2018. I mention 2019 as a remote possibility, since the details for Ice Lake for 2019 are quite sketchy right now, but I'm not optimistic for Ice Lake either. So maybe it's the following generation (2020) or later.
I couldn't wait that long so I decided to buy a 2017 MacBook, since Kaby Lake got 4K DRM support and 4K 10-bit HDR hardware decode support.