Just an update with my experience…
I have a free B2 account (i.e. the first 10gb) and I don’t have a payment method added.
The backup files are rotated and the ‘hidden’ files are removed permanently after a few days. I never have more than 7 files at once (Discourse has max 5 daily backups rotation on). Since my backups are never more than ~500mb each I’ve never had to pay for anything or delete any ‘orphan’ files manually.