Is it possible to encrypt a btrfs subvolume only subvolume only (no need need to be "transparent "transparent encryption")?
No, BTRFS does not currently have built-in support for encryption. Today Today, to encrypt a filesystem (not just a sub-volume) you'd need DM-Crypt/LUKS. See See the https://btrfs.wiki.kernel.org/index.php/FAQ#Does_Btrfs_support_encryption.3FBtrfs Wiki FAQ
From a person doing a minor link fix: I don't mean to be toxic, but f stackexchange. What do you mean edits must be 6 characters? The answer is fine, so you get this rant instead.