PD used to have more liberty, it's true, but the PD-Crim interactions in general were different. PD mains were very few back in the day, so most people playing PD understood what playing crim was like. Most didn't throw the book at everyone they ever arrested. A lot would act with what is fair rather than make arrests every time one came across their screen (a main example of this being white shirts minging and being killed after being told to leave places, although an arrestable offence, there was a time when basically no cop would arrest you for doing it).
The issue I think we face now when thinking about how to balance PD is that people have stopped looking at it as a "side faction" to make sure the streets are not just pure chaos and have actual risk, and instead look at it as a side to pick, which, for balancing purposes, becomes a much more difficult thing for PD HC to do.
The plain clothes detective rule isn't a "big deal" by itself, but I believe it reflects the change in mentality of many PD mains who now use PD as a tool purely to go after anyone they can and care less about the general health of the server and how they are affecting the equilibrium.