I will tell you this: there is a group of people who are committed to keeping Deis alive, called Team Hephy and the Deis slack is still active. I've used OpenShift and it is certainly more active, but I also have the very strong feeling that Deis is "done" where many parts of OpenShift look like they are still evolving. (And I mean "done" in a good way.)
If you are not sure, feel free to drop in and ask any and all questions: https://slack.deis.io/
The announcement of EOL came in July, and the announcement that Team Hephy would take over was almost one month to the day later, in August. I have a lot of faith in the Deis codebase and I'm on Team Hephy now.
We have yet to put out a fully e2e acceptance test passing and providence-verified signed, versioned release of our own, but we have and know of a number of production users that still use Deis in Production.
It definitely does not feel like a ghost town on the Deis slack. Questions get answered. Neither does OpenShift (it is definitely not deserted, and it does not seem to suffer from "abandoned k8s fork" syndrome, they do keep up with the release cadence, because they have to.)
I would go with the one that solves your problem better. I'm personally still recommending Deis, as I like the built-in support for Heroku buildpacks that OpenShift lacks. (Convox also has support for this, on ECS.) I'm not aware of any K8S-native Deis competitor (other than Hephy) at least for now.
Edit:
The really nice thing about working in Kubernetes is that you have the conformance suite[1]. You shouldn't worry about breaking something with the next Ubuntu update, because if Ubuntu update has broken Kubernetes in a way that will cause Deis to fail, then it will not be a Kubernetes-conformant distro anymore (and there is a large list[2] of K8S-conformant distros and platform to choose from, so you should pick a conformant one[3] instead.)
So, your question should be whether Deis will continue to work on Kubernetes distributions that are conformant, and with some minimal research, I think you should be able to say that with a high degree of certainty it will, regardless of ongoing vendor support from either side.
Then, you can ask whether your K8S distro is likely to remain conformant. (This should be a no-brainer.) Finally, is there anything that can break within Deis internally (not related to how it interacts with Kubernetes) that would mean you'd have to stop using it?
For me, the big reveal / final test will be, can we upgrade from Cedar stack to Heroku 16, which is on Team Hephy's planned roadmap for the future.
If that goes smoothly, then I'm not sure what else there is to worry about.
[1]: https://www.cncf.io/certification/software-conformance/
[2]: https://www.cncf.io/certification/software-conformance/#logo...
[3]: https://docs.google.com/spreadsheets/d/1LxSqBzjOxfGx3cmtZ4Eb...