The first thing that comes to mind is that debt stops making sense. In addition to your interest rate, your principle becomes more expensive over time.

A mortgage, a car payment, becomes more expensive over time since the deal was struck when prices were higher. A bitcoin can buy you a heck of a lot more now, but the mortgage you signed in 2014 was for 200 bitcoins. Uh oh...

Now, of course you can argue the necessity of credit until the cows come home. The bottom line is whatever credit is to you, deflation really throws a wrench in it.

Naively, other things the same, the best time to take out credit for something you need is now. Under a deflationary currency that time is never.