@Marian
This thing probably just maps the IOR to some insane value. What Dubcat is saying is that in the past if you wanted to have the IOR be really super duper mirror like (so that means an insanely high value) this kind of enabled you to go above the Corona IOR limit of 999.
That workaround has been removed now so there is no difference at all. 999 is the highest you can get.
Please feel free to correct me Dubcat.