Most of the time, if a mod is dropped, it's because it's either no longer being updated, or wildly unstable. I know Millenaire was dropped due to instability, at least, but not the others. Before we got 4096 block IDs working, there was also the chance that a mod would be dropped because it was redundant with another mod and the space was needed for something different.
I don't know if the changelog lists the why of a mod's removal, but it does list it, I think.