Okay, legality rules with Mystical Archive. While all the Mystical Archive is legal in Limited, Strategic Planning is legal in Standard as long as its normal variant is (see the Kaldheim version). All uncommons in this set are reprints of spells currently in Standard. The rares and mythics are from outside of Standard. Most of them are Historic-legal, except for these two and five others. Finally, that last card is legal in Legacy, Vintage, and Commander until Modern Horizons 2 releases, when it shall also enter Modern.
EDIT: Every Mystical Archive card will get a Japanese alt-art version. No idea when the'll show up, but supposedly they'll be from Wizards' Japanese offices.
I never get why they put Channel into these sets, like what is the point. It isn't legal anywhere because it's crazy busted, so what's the point of it, to deck out your cube with it (where Channel also tends to be a controversial card)?
I agree, people are overreacting to Channel, I mean T3-4 Colossal Dreadmaw would be a very strong play in Limited but that's about it, it'd take either a colorless monstrosity like an Eldrazi or an X-Spell to make it broken, else it's a ritual and that's about it.
Swords to Plowshares,on the other hand...
To make it clear I wasn't saying Channel is busted in this Limited. It almost certainly isn't as they surely won't have been stupid enough to put anything crazy broken with it in the set. I am more asking what's the point of its reprint, it has no real value, sees no real play because it's banned in everything so who wants this flashy Channel exactly? It seems completely pointless.