There isn't a reliable good quality source, as far as I know, but I can offer you a helpful answer, regardless.
Ask the question in a certain manner.
Here's my answer on this exact same topic:
The reason for why most developers in the eco-system write absolutely garbage code is because questions about process, patterns & implementation aren't asked and it's like the wild-west.
Almost all big themes, are built by people who aren't developers and just got lucky. Avada, Be, The7, Themify, Divi is actually pretty decent, while I don't agree with it, Genesis is "as clean as it can get for WP devs".
Trash. Nearly all of it.
This both means that you don't need to know how to code properly in order to achieve monetary success in this field and second (by contrast), you don't even need to care.
An interesting plugin to follow is POJO's Elementor. Although it's not SOLID (nor does it need to be) code, it's written in a pretty understandable manner. They combine OOP with procedural, not intentionally, but the result is an easy to understand eco-system.
ArrayThemes' themes are cleanly coded in terms of simplicity, but it's still hot garbage in terms of extendability, testing and such. I respect them for their stance on code and see where they're coming, but that code doesn't allow you to do much with it.
As such, you can see that a lot of vendors decide to do whatever they want with their code, because it doesn't matter and it's allowed. This is why both PHP / WordPress have a reputation for having bad developers and it's true. Remember that WP's functional approach to things is simply a remnant of the past and you shouldn't base your coding around it. Yes, they make use of classes with the newer features (2010+), but they're just patches.
I used the strong word "trash" but I respect these vendors, especially ArrayThemes for their stance on simplicity of their themes and code-base, but when you look at the code and you've got a bit of experience, you understand that writing your code in a functional matter, with classes that are 2000 lines each, you're never going to be able to extend that, except if you architectured it really nicely, which Elementor did.
If you can reason about your code and it's easily extendable and it's easy to predict, then you hit the jackpot, functional or not, OOP or not, even if it's "bad code by standards".
There is an engineer working on Avada that even though he doesn't write "good code", he's very technical and has deep understanding of the technical side of WP. If you look at their demos installer, you can see some arcane stuff that you never knew inserting a post could pose a problem to. There are some very talented developers in the eco-system who, if they learned about certain paradigms such as SOLID / TDD / Choices of procedural vs. OOP and think in terms of architecture, we'd have much better code bases.
Does this mean the world is going to end? Absolutely not, things still work, but have you ever tried extending any of these themes / plugins? It's hell.