I've been doing GIS dev work for a few years, but I "officially" became a GIS Developer last fall. I think a lot depends on the position and the organizational needs, but do note that the components of Portal can be installed on Linux machines. My org uses Windows, so I can't really speak to the difference between the two for Portal specifically.
All that said, I would personally try to dabble in both. I prefer to do a lot of my work in Linux, and being comfortable in that environment is very useful. We also don't exclusively use Esri tools, and certain other things we use are developed in a Linux-first sort of way, so it's good to be able to use those tools in an environment they were designed for.
Most Windows machines should let you run Windows Subsystem for Linux (https://learn.microsoft.com/en-us/windows/wsl/install), so even if you're stuck using a Windows machine for your work, you can still use that. There's also Docker, if you want to run a small throwaway Linux container on your machine to mess around. Some Esri products, like Notebook Server, are running Docker containers, so being familiar with them can be good to learn in general.
At the end of the day, though, most of my dev work is OS-agnostic. I am working with Python, or I'm using something like Experience Builder Developer Edition, or I'm doing something with our web GIS. All of these things run more or less identically on either system, the only difference being how they are installed and how their files are stored, or how I configure the process to be automated.
As far as general advice: just keep learning! Look at all the various GIS products out there, not just Esri, and see how they're being developed, what coding languages are useful to work with them, and think about your organization's present and future goals. See if your org can foot the bill for something like an annual Codecademy subscription. Trawl GitHub / GitLab / etc., for some interesting GIS projects and fork anything that catches your eye.
Get your hands on as many different concepts as possible and just fiddle around! When you find things that are particularly applicable to your work, dig in a little deeper.
- Josh Carlson
Kendall County GIS