It's a hand-waved estimate, but let's recognize that Apple actively plans on killing support for NAS targets for Time Machine:
> Time Machine backup to NAS devices over Apple Filing Protocol (AFP) is not recommended and won't be supported in a future version of macOS.
But that's AFP, not SMB. SMB is the future. [edit, that sounds sad].
AFP is what's deprecated, not Time Machine over networks. They just want you to use SMB.