Can Sett's passive ability be used on an opponent's turn to save a card, given that it doesn't have the Reaction keyword?

Ruling: Yes, Sett's passive can be used on an opponent's turn. It is not an activated ability but rather a replacement effect, so it does not require the Reaction keyword.

Nuances:

• The FAQ restriction about needing Action or Reaction keywords only applies to activated abilities that you choose to use
• Replacement effects function differently and are not subject to this timing restriction