Really interesting topic. I’d say for most testers I know, there’s no portfolio other than a CV(/resume) - however I can see some sense in illustrating complex testing problems with a documented approach, the output of exploratory sessions for example, gherkin etc… you could also document some of the conversations which happen around requirements, design and planning in order to demonstrate the kind of value the tester adds, even if that’s ordinarily explicitly “undocumented”.
I guess the problem I perceive in this is that testing is so inherently contextual, I don’t see a lot of employers devoting the time to understanding the business problem in order to then assess the testing output. The other thing is, testing is not undertaken in a vacuum - it is vital that testers are having conversations, asking questions, challenging assumptions. What can be documented is only half (maybe less) of the actual job, so that’s challenging too - but that’s not to say that other half isn’t also vital.
Overall I actually really like the idea, but I’d say that unfortunately in my vicinity, approaches and attitudes vary so much from company to company that it’s unlikely you’d meet someone who could meaningfully assess a portfolio in order to derive the value the idea implies. If your style of testing matches their understanding of it, or their aspiration for their team, great, but if it doesn’t? Sorry, not interested. The same is true of CVs of course… it’s a continuing frustration that we lack a way to meaningfully assess people’s ability to test well outside of actually sitting them in a team, which can be quite an expensive way of doing it.