The "Timeless Child" was a truth hidden by the Time Lords. A secret too devastating to learn, the discovery caused the Spy Master to ravage Gallifrey, apparently killing off the Time Lords. The secret involved the origin of the Time Lords and their founders. The Master implied the citizens of Gallifrey were told lies about this origin. The Timeless Child was buried in the memory all Time Lords. (TV: Spyfall)
The Remnants referred to the Timeless Child when drawing on the Thirteenth Doctor's fears and history on the planet Desolation. When they made the reference, the Doctor was stunned for a moment before she angrily told the Remnants to get out of her mind. (TV: The Ghost Monument)