No,more than half of the legendary Pokemon can be shiny.All of them can be found in wild,excluding Manaphy and Phione,which is only obtainable as an egg.
The legendary Pokemon that can't be shiny,which you can be sure that they are hacked,are Celebi,Arceus,Victini,Reshiram,Zekrom,Keldeo,Meloetta,and Genesect.