The Untempered Schism was mentioned in the Doctor Who shows: The Sound of Drums and The End of Time. Those were with the 10th doctor. Have any other doctors from the past mentioned the Schism, the ceremony where they make 8 year old kids look into it, or was this idea completely invented by the writers in 2008?
The series numbers are a little wonky since season 1 of "new-who" is actually season like 10 of Doctor Who makes things a little "fun" but Tardis Wikia on Untempered Schism shows in the history
When the Tenth Doctor's TARDIS brought him to ancient Gallifrey, billions of years before his time he and Cindy Wu witnessed young Gallifreyans being forced into the schism by a group of Time Sentinels. (COMIC: Old Girl)
Same article says in the Behind the Scenes section:
The Untempered Schism is similar to the caldera from The Book of the War, which is a place on the Homeworld that acts as a node point where all lines of historical influence converge and one can stare directly into the heart of time.
I can find no evidence of the Untempered Schism in earlier Doctor works.