When macOS Big Sur arrives this autumn Mac users will be able to back up using APFS - the file system that arrived in macOS 10.12 Sierra back in 2016.
The transition to APFS from HFS+ has taken a few years because initially it only worked with SSDs - meaning that Macs with Fusion drives (which combine an SSD with a hard drive to maximise storage capacity) couldn’t use the new file system. This changed in Mojave, and since Catalina all Macs have automatically been converted to APFS on upgrading the macOS. There was one element of the macOS that stayed in HPF+ though: Time Machine.
Right now a Time Machine backup uses the HFS+ file system, but in macOS Big Sur Time Machine backups will be in APFS, which may bring benefits such as taking less space up, being able to locate files quicker and more.
Another APFS related change coming to Macs (and iPads and iPhones) with the new suite of operating systems launching later this year is the ability to unlock and read the files on an encrypted APFS drive if you plug it into an iPad running iPadOS 14 or an iPhone running iOS 14 using a USB-C or Lightning cable.
9to5Mac has tested these features in the developer betas and claims that they already work.