I don't think the death rates have dropped significantly. Boxers still die, mostly in countries where the safety measures are not up to the standards that they should be.
http://ejmas.com/jcs/jcsart_svinth_b_0700.htm
Even in championship fights the occasional tragedy does happen, such as in the cases of Leavander Johnson, Akeem Anifowoshe, Gerald McClellan, Michael Watson...
In the 1950's and 1970's, no fighter died in a championship bout from what I know.