I had a pair of hubs with magnesium flanges, like 20 years ago. Of course there was the Rock Shox mag 21, it had magnesium lowers and brake arch.
I used to see magnesium bikes 10 years ago when racing. I'm sure they're still around but I'm out of the loop.
Fun story. A friend switched from cantis to V brakes way back when. We ground off the cable hanger on the brake arch. Of course we attempted to burn the filings. It was a strong white flame if I remember correctly.