Wow. Like across the board? That would mean no one could ever make progress with one session a week. But some people can make progress that way. Maybe not so efficient progress and for a certain period of time.
Now if you skipped the volume day you'd be very likely to lose some strength!
I don't think it's a good idea to completely skip the squats on the PR day. But I think how much strength, if any, you lose really depends on the individual and how advanced they are. Basically how heavy you lift.
I skipped whole PR days due to illness or whatever and basically just ended up with a lift that simply made me repeat the same weight on volume day, but no actual regression. Unless you think not loading the bar is acually losing strength. Which I guess could be a valid argument since you could say that you made gains but didn't keep them. But technically if you don't regress backwards you haven't lost any strenth for the WEEK, you have just failed to advance. What will happen for any one person can't really be predicted. In general though, it would be better to do something to maintain strength at least. On this setup what will generally happen, if anything happens is either you will have a really sluggish day next volume day, but still load the bar, or you will end up repeating the weight from last volume day.
The PR day is supposed to have you cementing the gains potentiated by the volume day. To say that you will drop a bunch of strength by not doing it is, I think, over-emphasizing it a little. You certainly shouldn't be skipping it a bunch and if someone didn't feel good about lifting really heavy that day they could maintain in other ways like I described.
*Edit* Very funny, D.
Now, I feel stupid ya bastard. .75 degrees stupider