After working as a programmer for about a decade, I went back to college to try to complete my education. This was when I first encountered code printed in a textbook. And it was just black monospace font on white paper. No color, not even bold / italic etc. It was Java.

Initially, it was much harder to read, both because of the habit I've developed and because, I think, the author also didn't use a "fancy" editor to write their code, so it wasn't formatted properly, didn't observe naming conventions. Well, your typical college professor writing. My first reaction was to type the code into the editor so I can explore it easier. This, of course, also provided syntax highlighting.

Of course, it took extra time. So, eventually, for shorter snippets, I'd just suffer, skipping all the typing part. I later discovered it to be somewhat helpful exercise when using Git diff or other Git commands that displayed code snippets in terminal w/o highlighting. But then the industry caught up to it, and now I don't have to read unadulterated Git output like that.

So, my conclusion so far: highlighting helps to discern the structure, to process larger blocks of code and to navigate it more efficiently. You can get better at reading code that's not highlighted, but it's never going to be as good as reading the highlighted code. It just seems like the struggle to even get to the level where it's palatable not worth it as there are very few situations where it would come in handy.