i've bought 2 wii's new that never came with this, but i bought both relatively late. maybe this only came with earlier wii's?
Google isn’t helping much but one might infer that the 2011 non-GameCube model dropped the part. I got mine in 2008 (or was it 09?) and it was included.
one of them is GC compatible, the red New Super Mario Bros bundle, the other one was the non-GC one though. i think i bought the red one around 2010, maybe 2011? i don't remember for sure though