Their linear channel was discontinued months ago. And it was a service channel. We can only speculate whether they paid carriage fee to Tata Sky back than or not.
They did pay carriage fee which is why it was available for free to subscribers. Yes, it was available as a service channel. There r many chs which originally run on satellite mode but r made available in indi as service channel. This has been a topic of discussion and rather many objections have been raised bcoz service chs do not require to obtain a license or pay any fee to the government. Maybe this is why Curiosity Stream was pulled out from Tata Play platform or they themselves decided to discontinue linear feed in India.
Anyways for now u can opt for this channel thru binge pack, on app or website i believe their subscription rates r quite high. Combo subscription access (online + linear) still not available in India bcoz here a different business model has been worked out