/usr/share/igtf-policy/slcs/DFN-SLCS.crl_url is in igtf-policy-slcs 1.88-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1
http://cdp1.pca.dfn.de/slcs-ca/pub/crl/cacrl.crl