Apple began phasing in the use of its SSD-friendly APFS file system with High Sierra for SSD-only Macs, and then upgraded Fusion Drive-based Macs in Mojave. In Big Sur, Time Machine volumes can finally be formatted with APFS, too. But in this transition, one capability was quietly lost: APFS volumes cannot be shared for network access via Apple’s relatively ancient Apple Filing Protocol (AFP).
AFP dates to the pre-OS X days, with a version appearing in System 6 in the late 1980s. As with most older protocols, it got long in the tooth, and Apple went from just supporting the Windows and Linux world’s SMB to shifting to it as the only built-in sharing method. Way back in OS X 10.9 Mavericks, Apple began moving away from AFP in favor of the industry-standard SMB, although it still hasn’t removed support.