I don't know based on the rulebook. The rulebook explains that when a Sanctuary is destroyed, its position can still be attacked, and the defending player removes the top card of their deck from the game, face-down (page 20). However, it does not specify whether attacking Unit abilities (such as Reaper) interact with the lost deck card in this situation. The rulebook does not define the Reaper ability or clarify how Unit abilities apply when attacking a destroyed Sanctuary's lane.

To answer this question, the rulebook would need to provide:

• A definition of the Reaper ability.
• Clarification on whether Unit abilities trigger when attacking a destroyed Sanctuary's lane and causing a deck card to be removed.

Source: page 20.

Source: Core Rulebook (pp. 11, 15, 20, 22, 24)