The set is not Standard-legal in paper either. It's a Modern-legal set, except for the cards in the "Lords of the Ring Commander" set, which uses a different set symbol, but comes in the same booster packs as the regular set. Those cards are not legal in Modern. You'd think that with "Commander" in the name, they'd also be legal in Commander, but one of the reprints is Karakas, which is banned in Commander. The rest of the Commander part of the set is, of course, legal in Commander, as well as Vintage. It's also legal in Legacy, except for Sol Ring, which is banned. Because the set was never Standard-legal, it's also not Pioneer legal. But it is Modern-legal, because Modern only allows sets that have been Standard-legal, except for the exceptions, which are also Modern-legal, except for the exceptions to those exceptions. Simple, right?
Oh, but that's just tabletop Magic. Arena doesn't have most of those formats. The Arena platform does have Standard but it also has the Alchemy format, which is like Standard, but ridiculous. There's also the Historic format, which is like the Arena version of the Pioneer format, except it has fewer sets because they haven't gotten around to working backwards that far yet, and is also uses Alchemy cards, which are not necessarily the same as the Alchemy format. Some Alchemy cards are legal in the Alchemy format, and other Alchemy cards are not legal in the Alchemy format, but they are legal in the Historic format. Oh, I guess that the non-Alchemy Standard is technically divided into two categories, the "Traditional" best-of-three version and the "Arena Standard" best-of-one version. Arena also has another technically-not-Pioneer format called "Explorer." Unlike Historic, Explorer doesn't use Alchemy cards. Supposedly, Explorer is supposed to eventually turn into Pioneer, but the card pool isn't all coded onto the Arena platform yet, and the employees who would presumably do that are busy putting Lord of the Rings cards into Historic-only Alchemy instead.
I love having a variety of formats in the game, but there are different ways to manage that. I think that some things have gone off the rails.
