Hi everyone,

There was a small update to the Comprehensive Rules today, mostly to add Battlebond to the Comprehensive Rules. But there was a fix for the Squee/Ixalan's Binding question:

601.3a If an effect prohibits a player from casting a spell with certain qualities, that player may consider any choices to be made during that spell’s proposal that may cause those qualities to change. If any such choices could cause that effect to no longer prohibit that player from casting that spell, the player may begin to cast the spell, ignoring the effect.
Example: A player controls Void Winnower, which reads in part, “Your opponents can't cast spells with even converted mana costs.” That player’s opponent may begin to cast Rolling Thunder, a card whose mana cost is {X}{R}{R}, because the chosen value of X may cause the spell’s converted mana cost to become odd.

This makes it so that Squee can no longer be cast if it's exiled by Ixalan's Binding. From Rules Manager Eli Shiffrin on Twitter:

Now you only look at information about the spell that changes while casting, not at information in general. You only get to ignore a “can't cast” if info about the spell can change while proposing it.

Examples of this would be converted mana cost (for instance, a spell with X in the cost) and card types (for instance, bestow).

Thanks,
Nathan Long
Magic Rules NetRep