According to the [[ https://www.ietf.org/rfc/rfc5023.txt | AtomPub ]] specification, this header is not mandatory :
> 9.7. The Slug Header
>
> Slug is an HTTP entity-header whose presence in a POST to a
> Collection constitutes a request by the client to use the header's
> value as part of any URIs that would normally be used to retrieve the
> to-be-created Entry or Media Resources.
>
> Servers MAY use the value of the Slug header when creating the Member
> URI of the newly created Resource, for instance, by using some or all
> of the words in the value for the last URI segment. Servers MAY also
> use the value when creating the atom:id, or as the title of a Media
> Link Entry (see Section 9.6).
>
> Servers MAY choose to ignore the Slug entity-header. Servers MAY
> alter the header value before using it. For instance, a server might
> filter out some characters or replace accented letters with non-
> accented ones, replace spaces with underscores, change case, and so
> on.
And the [[ https://docs.google.com/document/d/12O7p0lKtPFMn2JtOW3h8WDP0Miz9xKr7QSaHBMCbcGI/edit# | SWORDv2 ]] specify that:
> Server implementations MUST adopt the behaviour and requirements in Section 9.7 of [AtomPub] with respect to the Slug header
See also T2740, T2752 and ceb38e95a714