Proposing the rule of the golden cesspool: the closer code is to the heart of a business’s domain, the worse the code will be.
For example, at a travel tech company, the code supporting locations will be a nightmare. At a website dealing with books, the code around books and authors will be the gnarliest in the codebase.
There are several possible explanations for this phenomenon, but I favor the simplest: reality is messy, clean code is an approximation, and the closer your code gets to the hard and valuable realities your business is confronting, the less approximate that code is allowed to be.