Apple started shifting Macs from its old HFS+ filesystem to its more sophisticated APFS format with macOS 10.13 High Sierra. In that release, all SSD-based Macs were upgraded to APFS. Fusion Drives followed with macOS 10.14 Mojave. But it took a full three releases from the first APFS appearance until Time Machine backups could be written to drives formatted with APFS: macOS 11.0 Big Sur is the first release to allow that.
But you format a Time Machine drive to APFS, there are a few things to consider.
There’s no way to convert an HFS+ Time Machine volume to one that uses APFS without erasing it. Apple created an exceedingly wacky way to encompass Time Machine’s snapshot format inside the APFS format, almost as if the company hadn’t developed both APFS and Time Machine. (Insert thinking face emoji here, for sure.)
