Can't find the post to which answer, but a Mon being a Box Legendary is not a reason to not test it in OU. Kyu-B was in OU from Gen 5 to Gen 7, only when it gained DD + Icicle Spear it became Uber. Last Gen another Box Legend (Zamazenta Crowned) was tested in OU, though it remained banned eventually.
Don't have the energy to justify now tests for Zamazenta forms or Giratina, it's theorymoning anyway. When the time comes, I will discuss the potential influence or healthiness of both Mons in the context of the meta of that time.
Seconding this as well to an extent.
TBH, I think unless the council thinks that Zamazenta (either crowned or hero) would be a safe drop, I think it would be better to wait a little after HOME to test them. People are still grumbling about Wake, Garganacl, and Kingambit a bit, and HOME will be super turbulent. The only non Box legendary coming from Home I see getting on the initial banlist is Landorus I. We will have to contend with Urshifu S, Magearna, Spectrier, Regieleki, Ursaluna, Enamourus I, and Basculegion (before a last respects ban) who all seem very scary. I think Urshifu S, Mag, Spectrier and Last Respects will probably get the axe, but the point is once HOME drops, the metagame will be super turbulent as is.
Here is my shortlist of the at least fringe OU viable HOME drops.
Hisuian Arcanine, Zapdos, Galaran Zapdos, Moltres, Galaran Moltres, Mew, Galaran Slowking, Azelf, Heatran, Cresselia, Hisuian Samurott, Hisuian Lilligant, Hisuian Braviary, Tornadus T, Thundurus, Thundurus T, Landorus I (if it does not get quickbanned day 1 or not on the banlist originally) , Landorus T, Chesnaught, Hisuian Goodra, Hoopa U, Volcanion, Magearna, Rillaboom (losing grassy glide sucks but it still will be a decent wallbreaker), Urshifu S, Urshifu R, Regieleki, Spectrier, Kleavor, Ursaluna, Basculegion, Sneasler, Enamourus I, Enamourus T.
Obviously not all of those will be OU staples. I expect maybe half or a third of these mons will actually remain OU. The point is thought that we will be dumping a metric ton of viable mons into the tier. I think at least five in Landorus I, Magearna, Spectrier, Urshifu S, and Last Respects Basculgeion are very likely to end up being problematic and banworthy. You can even argue Regieleki or Enarmourus or whatever is also problematic, but I think those are more clear cut.
All those issues, on top pre existing issues in the tier, be it the three mons I mentioned above or lingering concerns about Terrastralization, make me think seeing if Giratina or Zamazenta or Origin Dialga or whatever is not a priority. I think those discussions are valid to have once the tier begins to stabilize. Once we get home, we have a relatively stable tier, maybe those mons are worth a look at the end of the day. I think personally Zamazenta Hero and potentially Crowned pending our Terra ruling could be less problematic for the tier than something like Magearna at least. I think it is important to at least talk about or consider these drops, even if it won't be a little while until they are feasible unless big papa Finchinator is confident he can drop Zamazenta from the initial banlist without it being on the radar immediately.