Yes, it works for free users now - at least it worked for me (I have since gotten an account.) I was using the Windows client via Wine, which worked surprisingly well until an update killed it a few months ago. During the same update, however, the Linux Preview began working for free users. I'd try it again if I were you.

It worked for me on ubuntu as a free user, even though the site said it wouldn't. I did experience issues with the ads getting stuck visible over the interface, but the audio worked fine.

That's surprising because I moved away from the Wine version specifically because the ads used to get stuck over the interface, even when Spotify was not in focus. With the Linux preview I never faced those issues again.