How can I delete my Stack Exchange account(s) and what happens to content I've posted when my account gets deleted?
The Help Center explains that there are two cases:
If your account has never voted and has posted at most once, you should see a self-delete option on your user page under "Edit Profile & Settings":
On some sites, it may look different:
If your account has posted more than once, or voted, go to the Contact Support page and fill out the form. One of the items in the topics drop-down list is for account deletion.
You can also get to that page via the "contact us" section on the site footer. Please note that account deletion is not instant. It might take several days for your request to be processed.
Due to a known issue, you might want to change your username before deleting the account.
Either of these procedures cover deleting your account on one specific site.
Content you've posted on the site is owned by you but licensed to Stack Exchange, and you therefore cannot request that it be deleted with your account.
Also of note, we automatically cull old inactive accounts as documented on the blog:
And, an important clarification, because this was lost on me when I was searching:
For those of us "don't delete anything unless you absolutely have to" types, "delete account" is synonymous with "leave community". Deleting an account does not remove your network credentials, it disconnects your profile from the other community.