Toxic boss keeps sending 'urgent' Slack pings at 8:30 PM. Do I ignore or cave?

My fully remote boss thinks "flexible hours" means 24/7 availability. Last night at 8:30 PM, my phone buzzed with an 'URGENT' Slack ping on a non-emergency issue. If I answer, I reinforce toxic behavior. If I ignore it, I risk my promotion. What is the play?