Calm will no longer be available on Sonos from January so suspect this relates to adding Calm as a service. From the grapevine Sonos are now starting to charge for music service integrations and Calm probably turned around and said sod that - this is hearsay and my take but probably not far from the truth.

Moderator edit: “From the grapevine Sonos are now starting to charge for music service integrations” - not even slightly true.

I’ve not heard this ‘grapevine’ rumor of Sonos starting to charge for service integration. Can you provide a source, please? I certainly haven’t seen anything here, either. 

Unable to send you a DM

You could post the link here. Should this new source of revenue not have been mentioned in the results Sonos has to publish? I’ve not heard about this.

No link, info obtained in a phone call from someone who works with both of the parties involved.

To be fair the developer site is gratuitously out of date, its effectively abandonware at this point, since it was made “pretty” a while ago the content has been frozen.

I noticed the moderator said it was not true about the fee

but is it true calm app will no longer be supported?

The moderators claim no knowledge of a lot of things posted here so imo take what they say with a grain of salt. 
but Correct Calm will be deleted from Sonos.