The Moodle admins of site complain that the certificates have lost logos. Unfortunately they took a long time to notice it. So I am not sure whether the Moodle upgrade 3.1 -> 3.5 did this or the subsequent server migration (Moodle release remained 3.5). Found a reference in the forum to moodledata/mod/certificate and indeed found pix/seals, /signatures and /watermark there.
When migrating Moodle instances I always take only moodledata/filedir with me. Is that insufficient? (By lucky chance I still have the old server intact, in two weeks it would have been too late!
Note: This is about mod_certificate, the old plug-in. Moodle version 3.1 upgraded to 3.5 and then migrated to a new server (all Debian GNU/Linux).