Actually, we have been phasing it out for a while. Long ago, it was the default section cut pen for Walls Int and Ext, but that was changed to 15. The same goes for the section cut pen for structural slabs and decks.
So I envision this having very little impact, but I'm giving plenty of warning anyway. I don't know how long exactly. Put it this way, in AC10 it will be official.
Why. We have used pen #1 as a section cut pen, which means it's heavy. Pen #1 turns up as the default pen in a lot of circumstances in AC, and its weight in our setup is not a good default weight in most cases. Therefore, I want to change the weight of pen #1 to something along the lines of pen #12. If you end up with pen #1 by accident, it won't be obnoxiously heavy. But if I change the weight, I need to be sure no one is using it for a section cut pen, or at least we're out of the habit.
So stop using pen #1 when you want a heavy line. Use 15, 25, 35, 45,... instead.