The dongle did not, in fact, materialize in 2017. But it does exist now, and it will go on sale this Friday for $80. The big, boxy adapter connects to the proprietary Surface Connect port on the current model Surface Pro and Surface Laptop (and only those systems; Microsoft says it's not compatible with any others) and offers a single USB Type-C port.
The port can be used with a Type-C charger to charge the computer (provided that the charger can deliver a minimum of 27W at 12V), or it can be used to connect Type-C peripherals to the computer. It supports USB devices, and also supports Type-C Alternate Modes to connect to displays using a Type-C cable.