Createrepo_c library doesn't support new erratum format which includes information about modules.
(UpdateCollection is not aware of module info at the moment: https://github.com/rpm-software-management/createrepo_c/blob/master/src/updateinfo.c#L88-L111)

Projects like Bodhi [0] or Pulp3 [1] rely on createrepo_c (Python bindings) to parse and generate updateinfo.xml. Without support of the new format later versions of Satellite/Pulp won't be able to generate updateinfo.xml for modularity data, and right now Bodhi can't produce a correct updateinfo.xml for modularity case as well.

The extended format is defined here: https://docs.google.com/document/d/1ePC9wlWrxlpAzruPkuIQyc-Zn_rFxRYr3Fj4NeewnGY/edit?usp=sharing

Example repo with updateinfo.xml with the new erratum format: https://repos.fedorapeople.org/pulp/pulp/fixtures/rpm-with-modules/

Example of a modular erratum pkglist in updateinfo.xml:

<pkglist>
    <collection short="">
        <name>coll_name1</name>
        <module name="kangaroo" stream="0" version="20180730223407" context="deadbeef" arch="noarch"/>
        <package arch="noarch" name="kangaroo" release="1" src="http://www.fedoraproject.org" version="0.3">
            <filename>kangaroo-0.3-1.noarch.rpm</filename>
             <filename> ... </filename>
        </package>
        <package ... >
             ...
        </package>
    </collection>
    <collection short="">
        <name>coll_name2</name>
        ...
    </collection>
</pkglist>

[0] https://github.com/fedora-infra/bodhi/blob/develop/bodhi/server/metadata.py#L28
[1] https://github.com/pulp/pulp_rpm/blob/master/pulp_rpm/app/tasks/synchronizing.py#L8