Helpfully, information has been rather drip fed, but more details have now been confirmed for Sky’s Freeview channels.
On Freeview, Sky Arts will be available on channel number 11, and will be broadcast on the COM5 multiplex, from the 17th September (2020) Sky Pick will move from channel 11 to channel 34, but will remain on the same multiplex. Depending on your equipment, a re-tune may be required.
It appears that Challenge is continuing, at the least for the time being. There is currently capacity on the COM5 multiplex, meaning that Sky Arts can be added without the need to close another channel.
Merit appears likely to close, once it has given Sky Pick the 34 channel number, but this has not been confirmed. Additionally, Sky Arts will also be available on Freesat channel 147.