A family friend (boy) got into Sidwell off the wait list for 4th a couple of years ago. He was moving from another private.

People do withdraw after the deadline, but the schools also have no obligation to then move to the waitlist. The year that my DS was admitted to a popular school in a nonexpansion year, we were told in May he would be the only boy joining the class. In September there were 4 more, but they were all kids who had been at the school previously, moved due to a parent's job change, and were now returning to the DC area. I don't think any had applied at the regular time.

There are very few withdrawals after folks lock into contracts. First question: Is the class overenrolled. If it is, that means more than one student has to withdraw or not commit. The anecdotes about summer waitlist movement might involve one applicant out of many grades at many schools. It's just what the numbers are.

holton...I think many schools end up having some spot--people move, can't afford it anymor etc

There is always a chance. The reason the wait can be long is it is really two parts. The first is that people at the school don't sign their contracts for the following year. So the school knows how many openings they have.
The second part is when tuition is due (and a financial situation changed) or a job transfer happens after the signing of the contract. A few spots can open up for this reason usually after June 1 when contracts become binding.

Has anyone had/know someone who has had experience getting off the waitlist at a DC Private for non-expansion years? I know that many schools won't necessarily take applications unless there is some sort of attrition. If such attrition actually occurs, how do the schools handle it and how long a process is it from that point on? Thanks.