When a card says 'abilities that add resources can't be reacted to' (like Seal of Rage or Kai'Sa Legend), does this mean you cannot react to spells paid for using those abilities?

Ruling: You can react to the spell itself (like Falling Star), but you cannot react to the ability that adds resources to pay for it (like tapping Seal of Rage). The "can't be reacted to" restriction only applies to the resource-adding ability itself, which goes on the chain and instantly resolves without allowing reactions.

Sequence:

• Opponent taps Seal of Rage to pay for Falling Star (this ability instantly resolves, no reactions allowed)
• Falling Star goes on the chain
• You can now react to Falling Star with cards like En Garde or Discipline

Nuances:

• The restriction only applies to abilities that add resources or casting permanents - these go on the chain and are instantly resolved
• This does not prevent reactions to whatever those resources are being used to pay for