[Update August 11, 2021] Spotify has now said that it will in fact support AirPlay 2. The original article is below.
Spotify ‘Decide to Close’ Working on AirPlay 2 Support
According to the post, the lack of Spotify support for AirPlay is due to audio driver compatibility issues” and that “this seems like a bigger project that we won’t be able to complete in the foreseeable future.” The post confirmed the streaming service had worked in this but has “decided to close it for now.”
The language of the forum post does not rule out the possibility permanently. However, it’s certainly not on the immediate agenda.