On Sat, Oct 6, 2018 at 1:33 PM KuboF Hromoslav <kubof.hromoslav@gmail.com> wrote:

The material is under CC BY 4.0 but Wikisource is using CC BY-SA 3.0. (so the problem is the version number) The argument is, that CC licenses are "forward compatible" but not "retro compatible". So it is OK to put CC BY 3.0 material into CC BY 4.0 repository, but not the opposite.
In the meantime I have found, that another Wikisources are collecting materials under CC BY 4.0, like https://en.wikisource.org/wiki/Category:CC_attribution_4.0

Note that Wikipedia uses many images under a variety of licenses, including CC BY 4.0 and fair use. An entire Wikipedia is not covered under any single license. Likewise, an entire Wikisource is not covered under a single license; https://en.wikisource.org/wiki/The_Grant_Shapps_Affair_Is_a_Testament_to_Wikipedia%27s_Integrity_and_Transparency is covered under the CC-BY-4.0 independently of any license the rest of Wikisource is covered by. It's mere aggregation, not making a derivative work.