Answering myself, https://fly.io/legal/sla-uptime/ says you get some credits for...

fulafel 3 months ago | parent | context | favorite | on: Fly.io outage – resolved

Answering myself, https://fly.io/legal/sla-uptime/ says you get some credits for under 99.9% uptime "provided that Customer reports to Fly.io such failure to meet the Uptime Commitment". So at least currently there's no talk of 99.99%.