What are Deis Workflow Alternatives?
According to the README of the deis/workflow project on GitHub, it is no longer maintained since March 1, 2018.

However, the README indicates that there is a fork, called Hephy Workflow. At the time of writing, this is the first time I heard about Deis and Hephy and I have not worked with these solutions.

Alternatives to these products are OpenShift, Google App, AWS Elastic Beanstalk, Azure App Service, Cloud Foundry, Apollo according to StackShare.

You mentioned that you are looking to build something internal. This would probably mean that native-PAAS like AWS Elastic Beanstalk, Azure App Service and Google App are not an option as these are only available on their platform.

One of the options could be OpenShift. This product has an enterprise and open-source version that can be installed on your on-premise or cloud servers. You could also decide to create a bunch of VMs on Google Cloud and install OpenShift on these servers.