Old Flash Player based work is absolutely still relevant in a portfolio or as a showcase. I would take the approach of having a screenshot or even a video demonstration of the project - with a link to the SWF. As Nancy mentions - people will increasingly not be able to view the SWF itself on a web browser - but that doesn't mean your work has no value.
Also - Windows 10 users can absolutely get Flash Player in all the ways they normally do - Windows Updates only applies to Microsoft browsers like Edge or IE11. It is part of Chrome and you can install for FF and others.