For some reason I thought the pricing page offered a way to pay CDCK to migrate a forum (e.g. from phpBB) to Discourse, but run the migrated site on a self-supported DigitalOcean instance.
Is that a product CDCK offers?
(If not, was that product ever available? Or am I just misremembering?)
Unfortunately not!
We’ve never offered that service. We will perform migrations for customers who are using our hosting plans though!
If I wanted to simulate the service, I’d pay for a year(?) of hosting, and then when the term is up I’d migrate from CDCK-hosted to self-hosted, right?
Yes, but obviously after a year of hosting you’d be so hooked on our awesome service that you’d never want to leave 
I haven’t yet created a product in my store, but you can head over to my site and ask me there, or post over in the marketplace. Include what forum it is, how many messages you have and how important it is to you that you minimize downtime between when you send the data and when you start the new site.