Like for a Facebook group.
With an optional message to the leaving member that existing posts, with or without username, will be kept.

I disagree.

Consider a member gets in a huff and decides to delete their account. Two things happen:

1) You end up with a lot of anonymised content in your community
2) The member cannot recover their account at all.

So if, when they calm down, they want back in, all their history is gone, and their posts unattributed to them.

Having an admin step in and evaluate it keeps the integrity of your community together and ensures that members don't regret their decision to remove their data.

2 hours ago, Matt said:

So if, when they calm down, they want back in, all their history is gone, and their posts unattributed to them.

Many thanks for commenting on feedback Matt.

That can be avoided by having a "regret" period in which the profile can be recreated. Like Facebook an many other services.

We cannot refuse a members request to be deleted so as it's working now it's just an unnecessary workload on the administrators and less privacy for the members.