2019-06-14 : After removing the last time counter, you must cast the card if able. Timing permissions based on the card’s type are ignored. If an effect prohibits you from casting the card, it remains exiled with no time counters on it, and it’s no longer suspended.

2019-06-14 : Crashing Footfalls has no mana cost. You can’t cast it unless an effect (such as that of suspend) allows you to cast it for an alternative cost or without paying its mana cost. Its mana value is 0.

2019-06-14 : Exiling a card with suspend isn’t casting that card. This action doesn’t use the stack and can’t be responded to.

2019-06-14 : You can exile a card in your hand using suspend any time you could cast that card. Consider its card type, any effect that affects when you could cast it, and any other effects that could stop you from casting it to determine if and when you can do this. Whether or not you could actually complete all steps in casting the card is irrelevant. For example, you can exile a card with suspend that has no mana cost or requires a target even if no legal targets are available at that time.

2021-06-18 : A card with no mana cost can’t be cast normally; you’ll need a way to cast it for an alternative cost or without paying its mana cost, such as by suspending it.

2021-06-18 : If a card with no mana cost is given an alternative cost equal to its mana cost (by Snapcaster Mage, for example), that cost cannot be paid and the card cannot be cast this way.