EDIT 2: Since many comments replied with the same thing, I'll reply here: I wouldn't consider read-only support as support for portable data storage, since that allows you only to send data in one direction. When you want to copy a file from a Mac to a Windows computer, NTFS is not a solution now (out of the box at least), therefore falls pretty short wrt "portable".
I thought it was FAT32. Do Macs support NTFS?
EDIT: It seems they don't out of the box: <https://www.howtogeek.com/236055/HOW-TO-WRITE-TO-NTFS-DRIVES...>
EDIT 2: Since many comments replied with the same thing, I'll reply here: I wouldn't consider read-only support as support for portable data storage, since that allows you only to send data in one direction. When you want to copy a file from a Mac to a Windows computer, NTFS is not a solution now (out of the box at least), therefore falls pretty short wrt "portable".