The feature has appeared on iOS devices for a small number of users, but appears to be based on a mode Spotify developed in 2017 for using the app while driving.
Spotify's statement is that it is "always working on improving the Spotify experience for our users, but we don’t have any information to share at this time.”
Tapping the magnifying glass icon (it appears in the centre of the bottom row of tabs) brings up a microphone icon inside a white bubble. Spotify then suggests examples of requests ("Play my Discover Weekly" or "Play some upbeat pop", for example).
This points towards the streaming company building its own ecosystem: reports have already begun that Spotify is building its own products (which could include a smart speaker). We've already seen dedicated Spotify players from third parties, such as the Mighty.
Since Apple's HomePod, the best-sounding smart speaker on the market right now, doesn't offer Spotify integration. It prefers to focus on Apple Music and Siri's role as a 'musicologist', which Spotify's assistant could undoubtedly challenge thanks to its access to data from (approximately) 140m users.
Moreover, with the company already removing integration from older hi-fi equipment, it's possible we could see it taking a new, more forceful approach when it comes to how it's integrated within other products like the Amazon Echo or Sonos One.