What OS should a master node CI virtual machine be? Say you are developing a new CI/CD system, deploy builds across Linux, Windows and Mac, and have now reached the point of wanting to deploy a master node which hosts your chosen CI tool. How do you choose what OS and version your master node VM should be?

For example with Gitlab, I can have the VM with Ubuntu, Debian or CentOS, or with Jenkins, the VM could be Mac, Windows, Ubuntu, Debian etc...

What's the major deciding factor when choosing an OS and version for the master node of the CI system?
I think the major deciding factor is the expertise of you and the rest of your company in the chosen OS. If you are a Windows shop, and your company is willing to leverage the cost of the node, it's probably best choice to host it on a Windows VM. For choosing a Linux distro, I would see what is most common across your company. In my opinion, it is not worth the benefit of creating a vm with a one off distro just because of a few particular features, unless those features of the distro are absolutely necessary.