Spotify confirmed to Engadget that the connection is now available for non-paying users and the streaming service's free tier is currently showing as supported on Home. Deezer, a streaming music service that's relatively new to the US market, is also now available on Google Home.
It's worth noting that Spotify's integration with Google Home will be different, depending on what tier of Spotify service you have. You can see a full list of available commands, depending on tier, at Google's support page, but while Spotify Premium users can play specific content on demand, free users are limited. You cannot listen to specific songs, artists, albums or personal playlists; instead, you can listen to one of Spotify's radio stations inspired by the specific song/artist/etc. you want to listen to. You can, however, listen to Spotify's curated playlists.
It's not ideal, but Spotify has been enforcing its tiers more as it's trying to renew licensing agreements and negotiate favorable royalty agreements. While the company had been long resistant to distinguishing between free and paid tiers it looks like the company is starting to chip away at services provided to unpaid customers.
Update: This post has been updated with confirmation from Spotify.