>It's not an expectation at non-startup tech companies.
Not true in the slightest. Before you get to talk to the engineers like this, you must first pass the HR filter.
HR are not engineers, but they think they see a parallel to their own work.
In HR, all HR applicants are expected to have a linkedin page, because linkedin makes HR's life a lot easier. I have Strong Opinions about this state of affairs, but none the less, there is a hovering perception in most HR offices of "if you don't have a linkedin page, how good at your job can you really be?".
So then HR hears the engineers talking about github and profiles and sharing code and they dig a bit, and they often decide that github is basically linkedin for engineers. Lets be real, they're kinda right in some ways.
But they're dead wrong in others. Lets not get sidetracked, but just say there are valid reasons some people don't use it. Engineers have the full picture and know this. Like you say, most professional engineers won't judge you for not having github.
However, HR lacks the context to understand this, and so they increasingly throw out engineering CV's that don't include a github link. It's a growing problem.