Skaro's time lock

Skaro's history was protected from potential Time Lord incursions by a time lock during the Last Great Time War. (AUDIO: Legion of the Lost) The Time Lords did the same with Gallifrey, defending it from the Daleks with a time lock. However, the Barber-Surgeon's campaign nearly broke the time locks around both Gallifrey and Skaro. When the Barber-Surgeon averted himself from existing, the time locks were both restored. (AUDIO: The Mission, The Horror)

Evidently, the time lock was no longer in effect following the War: the Twelfth Doctor unintentionally arrived on Skaro during the Thousand Year War, leading to him meeting Davros as a child. (TV: The Magician's Apprentice)