Shiny Locks only began in the Gen V games.
Shiny Locked
Gen V
Zekrom, Reshiram, Victini
Gen VI
Articuno, Zapdos, Moltres, Mewtwo, Xerneas, Yveltal, Zygarde, Groudon, Kyogre, Rayquaza, Deoxys
Sun/Moon
Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini, Cosmog, Solgaleo, Lunala, Nihilego, Buzzwole, Pheromosa, Xurkitree, Celesteela, Kartana, Guzzlord, Necrozma
Ultra Sun/Ultra Moon
Zygarde, Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini, Cosmog, Solgaleo, Lunala, Necrozma, Zeraora
Let’s Go Pikachu/Eevee
Starter Pikachu and Eevee
Sword/Shield
First encounters with Grookey, Scorbunny, Sobble, and Galarian Slowpoke (all of which can be bred to get shinies), Eternatus, Zacian, Zamazenta, Type: Null, Kubfu, Cosmog, Poiple, Calyrex, Glastrier, Spectrier, Keldeo, and the Galarian forms of Articuno, Zapdos, and Moltres (their Kantonian forms are not locked).
Serebii
However, before the Gen V Games there were still unobtainable Shiny Pokemon, as a result of being directly distributed from events. These are not technically "Shiny Locked" but I'll list them below;
Gen II
Mew
Gen III
Celebi
Gen IV
Spiky-eared Pichu, Arceus, Celebi
Gen V
Celebi, Arceus, Keldeo, Meloetta
Generation VI
Cosplay Pikachu, Celebi, Arceus, Victini, Keldeo, Meloetta, Diancie
Gen VII
Cap Pikachu (except Partner Cap), Victini, Keldeo, Meloetta, Hoopa, Volcanion, Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma, Magearna, Marshadow, Zeraora
Gen VIII
Zacian, Zamazenta, Eternatus, Kubfu, Urshifu, Zarude, Calyrex, Glastrier, Spectrier, Galarian Articuno, Galarian Zapdos, Galarian Moltres.
This also factors in that some Pokemon can only be received in a particular game by trading from a previous game, in which they may have not been available as a Shiny.
Bulbapedia
