Here is a list of the unobtainable Pokemon through Gen 8:
Articuno, Zapdos, Moltres, Mew, Raikou, Entei, Suicune, Lugia, Ho-Oh, Celebi, Jirachi, Deoxys, Uxie, Mesprit, Azelf, Dialga, Palkia, Giratina, Heatran, Cresselia, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Cobalion, Terrakion, Virizion, Keldeo, Meloetta, Genesect, Reshiram, Zekrom, Kyurem, Xerneas, Yveltal, Zygarde, Hoopa, Volcanion, Magearna, Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma, Type: Null, Silvally, Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini, Nihilego, Buzzwole, Pheromosa, Xurkitree, Celesteela, Kartana, Guzzlord, Stakataka, Blacephalon, Marshadow, Zeraora, Meltan, Melmetal, Zacian, Zamazenta, Eternatus, Regieleki, Regidrago, Zarude, Glastrier, Spectrier, Calyrex, and Enamorus.
Every Gen 3 legendary is available except Deoxys.
Additional locations of some select gift/static legendaries are on
this sheet.