Cannot load rsync daemon in Sierra. Log shows: "assertion failed: 16G1408: libxpc.dylib"
Apparently, Sierra and possibly earlier versions as well, need the rsync daemon to be started with the --no-detach option, which wasn't needed before.

The error message assertion failed: 16G1408: libxpc.dylib that appeared in the logs when it didn't work remains a mystery. But this .plist file now works to solve the original problem:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;Disabled&lt;/key&gt;
    &lt;false/&gt;
    &lt;key&gt;Label&lt;/key&gt;
    &lt;string&gt;org.samba.rsync&lt;/string&gt;
    &lt;key&gt;ProgramArguments&lt;/key&gt;
    &lt;array&gt;
        &lt;string&gt;/usr/bin/rsync&lt;/string&gt;
        &lt;string&gt;--daemon&lt;/string&gt;
        &lt;string&gt;--no-detach&lt;/string&gt; &lt;!-- This is needed now --&gt;
    &lt;/array&gt;
    &lt;key&gt;RunAtLoad&lt;/key&gt;
    &lt;true/&gt;
    &lt;key&gt;KeepAlive&lt;/key&gt;
    &lt;dict&gt;
        &lt;key&gt;SuccessfulExit&lt;/key&gt;
        &lt;false/&gt;
    &lt;/dict&gt;
&lt;/dict&gt;
&lt;/plist&gt;