SteveEricJordan wrote:
I never noticed a change myself
You probably didn't play
very old maps. Play any map from 2007 or 2008 (
this one for example) and you will understand.
The following is how I see the evolution of beatmapping until 2012 (from a non-mapper's eyes).
Basically, in 2007-2008, there were no (or very few) rules about beatmapping so the general quality was quite low. There were inconsistent jumps and spacing, a lot of circles, few and ugly sliders and sometimes ninja spinners. Circles were often very small and followed the logic of the Nintendo DS games Ouendan and Elite Beat Agents (they followed for example the voice more than the beat). Timing was sometimes hazardous.
After that until about 2011-2012 I think, osu! gradually got its own mapping styles thanks to some beatmappers who led the way (val0108 is the first that comes to my mind but there are others). AR8 was the rule, AR9 was not that common and AR10 was practically non-existent. Focus was put on jumps, circles were made bigger (maybe not as much as nowadays), sliders were better made and more common thanks to an improved editor. New rules banned some mistakes from the past, like ninja spinners.
After that, I can't really tell since I played less but all that I know is that at some point, focus was significantly less put on aim and more on speed and reactivity, especially with higher ARs (particularly AR9+; AR8 gradually disappeared on Insane maps) and higher BPMs.
About the evolution of streams, I can't tell.