This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The Interdimensional Displacement Restriction was part of the Temporal Accord which dealt with travel between universes.
After Yor began experiencing a unique medical condition as a result of travelling both through time and across universes, and the only known cure was to have returned him to his place of origin. Despite the severity of this condition, the United Federation of Planets refused to send him back as doing so would break the restriction which was considered to be "ironclad". His doctors then petitioned for euthanasia instead. (DIS: "Terra Firma, Part 1")