A flaw was found in the way certificate signatures could be forged using collisions found in the SHA-1 algorithm. An attacker could use this weakness to create forged certificate signatures. This issue affects GnuPG versions before 2.2.18.
mdeslaur | in master, gnupg disables SHA-1 signatures completely. In the 2.2 branch, it disables SHA-1 signatures after a certain date only.