See the information in Types of disks you can use with Time Machine on Mac - Apple Support including the following:
APFS or APFS Encrypted disks are the preferred format for a Time Machine backup disk. If you select a new backup disk that’s not already formatted as an APFS disk, you get the option to erase and reformat it. If the disk is a Mac OS Extended format disk that contains an existing Time Machine backup, you aren’t asked to erase and reformat the disk.
Note: The entire APFS volume is reserved for Time Machine backups. If you want to store files other than the Time Machine backup on the same physical device, use Disk Utility to create an additional APFS volume on the disk. The two volumes then share the available space.
Time Machine still supports backups on Mac OS Extended format (Journaled), Mac OS Extended (Case-sensitive, Journaled), and Xsan formatted disks.