For example, check this question here, the account of the OP is not clickable, and appears greyed-out:
Why this is the case for some accounts? Are they deleted? If yes, how can one delete their own account? Or can only the admins delete accounts? And for what reasons can accounts be deleted?
Yes, the account in question has been deleted. (In general, accounts which are greyed out either belong to a now-deleted account, or, in the case of migrated questions, possibly to an account that was never created. When an account is deleted, the display name is reverted to the default user#### form.)
Voluntary account deletions can happen in basically two ways:
Of course, from time-to-time moderators discover accounts that have been created with nefarious motives (frequently sock-puppetry or suspension circumvention). When such accounts are discovered, they are usually unceremoniously deleted by the mods. Accounts created with the purpose of posting spam and other nonsense are simply destroyed by moderators when found, which has further implications.
