fa07ec931b97227229528f962eb52646f1e80d51 sets git config core.logAllRefUpdates = false on the zuul merger. Looking at the repositories the reflog is still appended.
Rereading git-config(1) it is false by default in a bare repository, however if there is an entry in refs/logs/ it is still happening.
My theory (to be checked in the source code or empirically tested) is that deleting refs/logs/* would prevent them from being created with core.logAllRefUpdates = false