Can a player exhaust all their runes in response to a reaction speed spell that will kill their Ekko unit, even if they don't have costs to pay at that moment?

Ruling: Yes, you can exhaust your runes in response to the spell. Both the exhaust and recycle abilities of runes are at Reaction speed and can be used any time a reaction can be made.

Sequence:

• Opponent casts reaction speed spell that will kill Ekko
• You can exhaust all your runes in response at Reaction speed
• Once the spell begins resolving and kills Ekko, you don't have another moment to react
• The exhausted runes recycle when Ekko dies, giving you additional energy for the rest of the turn

Nuances:

• Rune abilities can also be used mid-resolution when a card asks you to pay a cost, though this is not relevant to this scenario
• The benefit is gaining more total energy for the turn (e.g., with 8 runes where 3 are exhausted, exhausting the other 5 before Ekko dies gives you 13 effective energy for the rest of the turn)