I wonder if Fly.io still waves invoices that are less than $5, most part of the internet says so but there seems to be no policy talking about it and also things saying it discontinued after October 2024.
You’re right to treat it as iffy and not necessarily permanent, overall, though. As just a fellow user, I’d recommend not making any plans that rely on it continuing indefinitely into the future.
You can email billing@fly.io to get the official word on this and any other payment policies, . Unlike the other addresses, you do not need a Support plan to send questions there.