And open source contributions are a lot easier to make than you might expect, when you consider non-code contributions. Opening an issue on github for an open-source library is a great signal. A well-written bug ticket tells me:
1 - You write enough unique software to encounter bugs in other developers' work (i.e., you're not just duplicating the same CRUD work everyone else is)
2 - You understand how to identify common tasks in your application that should be delegated to a library (and don't try to just re-write everything yourself)
3 - The details in the bug ticket show that you understand what information is relevant to solving a problem and can communicate with other developers.
Those sorts of contributions should be possible in most work environments, as part of your normal job responsibilities.
(I do have Githubs, but they are because I like coding for fun, and I don't like coding for fun to find jobs.)
I wonder how one can prove to have a track record of getting stuff done. If we exclude hobby projects / open source contributions, I have no idea how I could show any kind of track record. I'm not allowed to talk about the projects I do at work, for example, and I don't have time or motivation for hobby projects after work.