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.

Could be. Slackware is a popular name, but I would call it "niche" rather than a major distribution. Just my personal view, obviously.

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.

It always gives me a spell of cognitive dissonance when someone points out that Slackware is no longer a "major" distribution.

It used to be the major distribution. Funny how times change.