Hadn't heard of it before, and it appears not to be.
There indeed exist a few non-Vixie-cron-derivative implementations but as far as I'm aware, all major Linux and BSD distributions use a Vixie cron derivative.
Edit: I see now where I caused confusion. In my original post, I should have said all default cron implementations.
I thought Dillon cron was the default cron in Slackware? Hard to be a more major Linux distribution than Slackware, in terms of historical impact if not current popularity.
I just confirmed with a Slackware user today, it still does use Dillon cron. I had a vague memory from before I switched from Slackware to Debian late last millennium.
There indeed exist a few non-Vixie-cron-derivative implementations but as far as I'm aware, all major Linux and BSD distributions use a Vixie cron derivative.
Edit: I see now where I caused confusion. In my original post, I should have said all default cron implementations.