This is true and not true.
In PvE, i bring the code from Tank aggro in. The pet can tank 10 mob around you and you rarely get hit therefore PvE disengage is not needed. (trust me, pet (all 3 spec Ferocity/Tenacity/Cunning) can tank everything without the need to trap adds).
Also Combat Routine use intelligent cool-down to keep pet alive including: Mend Pet heal, Spirit Mend heal, Cower (reduce 40% damage), Last Stand (instant heal pet for 30%), Exhilaration (instant heal pet to 100%), Heart of the Phoenix (instant revive pet), Dismiss Pet (when pet about to die) and Revive Pet... I think that enough to keep pet tank big group of add
In PvP, there certainly Disengage built-in that activated when Melee come next to you.