It was never a very comfortable or nice solution, but so far you could use Spotify in Safari without problems. Now it is probably over, presumably because the used Widevine plugin for the copy protection of Apple is not more than surely recognized. Spotify itself did not comment on the problem and it is probably not clear whether it was now, or the feature somehow reactivated. As before, you can of course switch to other browsers or just to the app.
Apple’s interest in ensuring the smooth running of Spotify is to be relatively low, since they are indeed in direct competition with Apple Music.