I know of a few startups with relatively 'senior' people who just don't care at all or are seemingly unaware about 'best practices', and will happily write tens of thousands of the most messy code, neglecting standardizing anything or refactoring their code to be more modular or easily understood.

It does seem to be something the big co. interview gauntlet is designed to weed out, whether it's worth it or not. I'm sure it's still relatively common, but probably a lot rarer than at the median startup.