It's still relevant - there are enough differences between the browsers to render incompatibilities and bugs. It does also depend on how serious you are about

It's still relevant - there are enough differences between the browsers to render incompatibilities and bugs. It does also depend on how serious you are about your product, as some kinds of products not working in a browser can leave a very bad or frustrating experience. It is of course painful to test in so many variants, so a decent compromise is often to just test in the latest version of all browsers. There's enough coverage there to help you find any potential problems.