That's something it's really difficult to get data on it seems. I'm not even entierly sure when same-day weigh-ins were stopped. I think it was around 83-84, but I don't know exactly - and even then I don't know how strictly it was enforced. After that you need info on all the severe boxing injuries - it's too much.
Yeah it is difficult to prove conclusively that the change in weigh-in days has made it safer. The rate of ring deaths hasn't markedly decreased over the last couple of decades: http://ejmas.com/jcs/jcsart_svinth_b_0700.htm However there's not a whole lot of info regarding non-fatal serious injury so assessment of the merits of the 24-hour weigh-in difficult. Personally I'd prefer a return to same day weigh-ins rather than what we have today where there can be a great disparity between weights on fight night.