Thinking about shiny locks just now and it seems to me that GF is kind of inconsistent with how they're applied.
- In Gen V, only Reshiram, Zekrom, and Victini are shiny-locked. All other catchable legendaries - Kyurem, the Kami trio, and the Swords of Justice - are available in their shiny forms. The three mythicals are obviously also shiny-locked by dint of only being available by events. Additionally in B2W2 all of the old legends are available shiny.
- In X and Y all legends and mythicals are shiny-locked, period. In ORAS Groudon, Kyogre, Rayquaza, and Deoxys are shiny-locked but everything else is available as a shiny.
- In Gen VII Solgaleo, Lunala, Necrozma, Cosmog, Zygarde, all the Tapus, all the Ultra Beasts, Ash-Greninja, and all the mythicals are shiny-locked. In USUM, this no longer applies to the Ultra Beasts (except Poipole) and does not apply to any of the older legends available via the Ultra Wormhole.
- In Gen VIII Zacian, Zamazenta, Eternatus, Cosmog, Kubfu, Calyrex, Spectrier, Glastrier, Keldeo, and the Galarian forms of the legendary birds are shiny-locked. In addition, the starter trio and the scripted encounter with Galarian Slowpoke are all shiny-locked too, but can be bred to produce shiny forms.
Few things jump out at me there:
-In BW they didn't lock everything they could have. You'd have expected Kyurem or possibly even Landorus to be shiny-locked but it seems like initially they just wanted to lock the cover legends as well as Victini, which is a mythical. For obvious reasons, Game Freak seems to dislike giving easy or early access to shiny mythicals since Mew and Celebi were always distributed with fixed IVs in Gen I and II - while Celebi could be SR'ed for in Crystal, this was only ever possible in the Japanese titles until recently. Mew wasn't available shiny until Gen III, and only then on Japanese copies of Emerald with an extremely elusive event item. Consider that in 2021, eleven years after BW's initial release, Meloetta, Keldeo, and Victini are still unavailable in shiny form; there has never been an event for any of them, despite ample opportunity.
-In X and Y there isn't a "lower" legendary trio so they just said "screw it" and locked everything, including Mewtwo and the three legendary birds. I suppose they locked Zygarde because at this point they expected another Kalos game to happen and probably anticipated that they could use shiny Zygarde for an event. Part of me suspects they regretted giving early access to shiny Kyurem in BW and wanted to keep something back this time around.
-By the time we reached SM, it looked like they wanted to lock ALL special Pokemon, period, so not one of the catchable legends can be caught shiny. Except... did they forget about Type:Null? It seems an odd one to leave out given that the game is clear on it being a legendary. Yes it's a pain to SR for but people will do it. They later made a big hoo-hah about releasing shiny versions of the light trio, Poipole, and the Tapus, so you'd think they'd do the same with Type:Null.
-In Gen VIII they've again just locked off absolutely everything, and even made it so you can't get a shiny starter (which seems unduly harsh). Oh, wait. Turns out Regieleki and Regidrago were never shiny-locked. I'm not complaining, but... why not? Those two, along with Glastrier and Spectrier, are four of the most wanted Pokemon in the game right now. Why wouldn't GF take the opportunity to restrict them?
Serebii has a good list of all the shiny-locked Pokemon at the time of writing - there's some oddballs on there, like shiny Fancy Pattern and Pokeball Pattern Vivillon or hat Pikachu. What intrigues me is that there are some Pokemon, like Zeraora, for which it only took GF two years to release the shiny for, and yet others, like Victini and Hoopa, that we are still waiting on after years and years. Keldeo is encounterable for the first time in the Crown Tundra, and yet GF still shiny-locked it. Zeraora has no connection to Galar, and was an odd pick to be the prize for "one million victories" (seriously, why not Victini, it's literally there in the name) - did GF just look at all the mythicals and pick it randomly? Or is there more of a reason to it?
What do you think is the reason we haven't seen some of these yet?