I wanted to weigh in on this topic as there's some redundant information being spread here.
Basically the answer to the question "does erasing my iOS device via 'Erase All Content and Settings' securely wipe my data?", is yes and no.
First let's start by explaining how deleting a file deletes a file on pretty much all platforms with few exceptions.
All filesystems have one form of a file index, there's many different types of this, but in essence they're the same. The index basically says "IMG_1234.jpg" is located on these sectors of the storage device. The index is used to quickly locate files on your device. When you delete a file, it simply removes the file from the index. But in almost all cases it doesn't go to the actual section of your storage device and overwrite the data itself, it just deletes the index, which for all intents and purposes makes the file go away.
It's widely known however there are a plethora of free software titles that will basically ignore the index, and scan each sector of your storage device and give you the ability to recover the data. Software like this can indeed recover files you deleted.
In order to securely destroy the data itself, not just the index, you can use other software that will not only erase the index, but will go to the sectors on the disk, and actually write random 0's and 1's to the sectors, sometimes several times, or 'passes'. This is the only way to securely erase data. This is secure, but also much much much slower.
When you "Erase All Content and Settings" on your iOS device, basically what it does is recreate the index - not actually erasing the device by writing 0's and 1's over the top of the existing data. We know this because the erasing takes seconds, whereas secure wiping will take hours - simple math. When your device reboots, it looks brand new because the index has been recreated.
So technically no, it's not a secure wipe.
---- BUT ----
And I mean a huge but. All iOS devices, for many years now are encrypted by default, in fact unless you go against the recommended settings to enter a passcode and passphrase, all content on the device is AES256 encrypted, with the exception of some files, but certainly your data. Encrypted files makes it almost impossible to decrypt the data without your decryption key. I say almost liberally, I basically mean impossible.
When you do an "Erase All Content and Settings" the iPhone will securely destroy the decryption passcode and passphrase, rendering the data still technically on your device, but impossible to recover.
To ensure your device is protected you can go into "Settings" > "FaceID & Passcode" / "TouchID & Passcode" and scroll all the way to the bottom, in text form you'll see "Data protection is enabled". If you see this, then you'll know the above information is accurate. If it's disabled, use a passcode.
To put this to the test I erased my iPhone 7 plus as I just got my X and used several 'recovery programs' on my Windows PC, and was unable to recover any data whatsoever.
So in summary:
"Is it a secure wipe?" - Not technically.
"Is your data recoverable by any means?" - No, basically impossible.
Caveats of course include local iTunes backups and iCloud backups. While the data isn't recoverable from the device, data can certainly be recovered from backups - but that's a topic for another day.
Hope this helps.