Deleted photos aren't when it comes to iOS, a problem exposed by a bug which resulted in photos being restored after users deleted them. The Verge is unequivocal about the pressing quality of this problem: "Apple needs to explain."
This is obviously a privacy concern. It raises valid questions as to how Apple stores photo data and whether iPhone owners can truly trust that their deleted data is actually deleted. The Verge has reached out to Apple multiple times to comment publicly on the matter but has yet to receive a response. Doing so would at least shed light on why this bug happened, what's been done to fix it, and what it's doing to ensure that this won't happen again. However, Apple has yet to respond.
What's troubling is that, so long as Apple remains silent, we have no idea of how far this bug goes. Some iPhone owners have reported the same thing happening with deleted voicemails. Did the bug only impact people who use iCloud photo backups? Another post claimed that old photos appeared on an iPad that was sold to another person. All today's fix confirms that this bug did exist, it was a problem, and it had something to do with database corruption. And by ignoring requests to comment publicly on the matter, it doesn't impart confidence that this won't happen again.
The problem with privacy-as-a-product is that it comes with a license agreement and you haven't read it.
Update: Removed "Worse, this reportedly has happened on phones which had been sold to new users" as the claim was uncorroborated.