Time Machine disk running out of space

[jchuzi]

TechToolProtection notified me that my Time Machine disk has only 15% available space. In the past, I have erased the disk and redone TM, and that is what I intend to do. I have forgotten how to format the disk for TM. Should it be Mac OS Extended or APFS? I recall that the former is correct but it has been a few years since I had to do this. Disk Utility lists Mac OS Extended as the default for erasure.

[jchuzi]

In another forum, a poster said that Time Machine now requires APFS for snapshots.

ADDENDUM for those who want to set up TM:  It turns out that the correct format for Time Machine is APFS (Case Sensitive). If you start an initial TM backup with a different format, TM automatically will reformat itself to the right one.