For all practical purposes SATA Express appears to be a dying technology (if not already dead). M.2 connectivity seems to have sounded its final death knell. It was originally designed as a SATA interface for devices that would be specifically designed for SATA Express but they never materialized since SSD manufacturers (by & large) exhibited no interest in this technology. And then came M.2...
It's conceivable I suppose that more interest may now be generated in SATA Express since ASRock has a front panel USB 3.1 adapter that utilizes the SE connectors that's included with their high-end Z170 Express 7+ motherboard and also sold separately. But I wouldn't count on it.
Anyway, a user can still utilize the SE ports for connections to...