If it's been withdrawn from the 'free to air' channels transmitted from the Astra satellites (which Freesat simply offers a subset of) then I'm surprised that nobody has updated the relevant Wikipedia page, which still lists it:
https://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E
I'm further surprised that it's not been removed from the channel list on the official Freesat website:
http://www.freesat.co.uk/channels?
Most of all though, I'm surprised that Freesat is still listed as a platform on the Challenge TV website!
http://www.challenge.co.uk/
So my guess is that it's just a temporary glitch. (Unlike some other satellite channels, Challenge TV is actually owned by Sky. So it would seem unlikely that they'd withdraw the channel from the Astra satellites unless, for example, they needed to free up a channel for use by a new service).