I noticed something interesting that explained the break in period for trucks/bushings (at least for me I think) because I could never wrap my head around how exactly it should work.
The observation: I have 2 setups with the same truck configuration (Ace AF1 where I swapped out the front barrel bushing with a 88a Independent), but one setup I use to ride around everywhere (including rainy days) while the other is just for going to the bowl, so it gets used less.
The bowl setup rides really flowy, the everyday setup is much more stiff in comparison even tho the kingpin nuts are exactly the same height etc. Only difference is the age and how dirty both boards are.
Out of curiosity I cleaned and greased the kingpin and pivot cup on my everyday setup and now it rides as flowy as the other board again.
So my thoughts on bushings "breaking in" is: maybe it's just dirt/dust settling in between the surfaces so they no longer slide around as easily. I mean of course you can see the bushing deforming a little bit, but the main thing that does is probably getting better contact to every surface they touch. The bushings themselves don't really get harder, it's just that they resist the turning with the twisting too.
For me it finally makes sense.