I think I understand the stated issue that people are finding ways to broadcast transactions out of band directly to miners and that this "is bad for decentralisation".

But how exactly?

The only thing I can think of is that it becomes harder to know if censorship is taking place if you have no idea what you expect to see in a block because half of the backlog is private. So potentially it affects censorship resistance.

But what other effects are you referring to? How is this limit reducing decentralisation?

If you already have a writeup, link it, I'm just trying to understand.