How long before WWDC are old models usually put on sale? I used to work as a salesperson at an Apple retailer. I remember that prior to WWDC Apple usually changed the target price on old computers before the new computers were launched. (I guess they are easier to sell before the new models come out). Are there any statistics when this usually happens and how much those prices change?
I am not aware of public price cuts prior to WWDC. At least not with today's Apple.

The practice you mention may no longer be in place, it may be limited to some stores, or it could be country/region specific.

Apple's computer products tend to maintain the same price for their lifetime. iOS devices can attract discounts over time – their more predictable update cycle helps with this.