The only metadata stored in the clear in bcachefs is the superblock and the very first part of the btree node and journal entry headers - just the checksum, mag

The only metadata stored in the clear in bcachefs is the superblock and the very first part of the btree node and journal entry headers - just the checksum, magic number (identifying it as a btree node/journal header), and a flags field which mainly contains the checksum/encryption type. So an attacker can tell roughly how much metadata you have, but literally nothing about the contents.