To add a bit more context here: Malte probably meant this in TarSCM/changes.py
126         if service is None:
127             service = ET.fromstring("""\
128               <service name="tar_scm">
129                 <param name="url">%s</param>
130               </service>
131             """ % url)
132             root.append(service)
133             write_servicedata = True

but this class parses the _service file, which can be used directly to inject malicious XML.
 73             return ET.parse(servicedata_file, parser=xml_parser)

a good description of what is possible with this:
https://pypi.org/project/defusedxml/

In this uses case I see this as a hardening mostly. Because of that I'll reduce the severity. Tracked in https://github.com/openSUSE/obs-service-tar_scm/issues/382 for visibility

this is still valid and unfixed

the network protection is handled in the network setup of build.opensuse.org server.
Not sure how any generic filtering of host names would look alike...