There will be absolutely no difference; It will just keep on working like it always has. Time Machine under Mojave doesn't even support APFS anyway; A Time Machine backup will always be made on an HFS+ type sparse bundle (APFS in Big Sur). -
Also think of it this war. If the communication is done properly, it doesn't matter what file system is on the other end. It will be transparent. My webserver runs on an EXT4 filesystem. macOS and Windows cannot read this by default, but my web pages render just fine on both, because the communication is transparent to the underlying file storage.