Using Non-Mac Computer to delete iPhone files

iOS doesn't lack such a basic feature. It's intentional.

Being able to delete images or other data on an iOS device simply by connecting to any external computer would mean it would be very easy to create apps that can go beyond file deletion to modifying apps or the OS itself.

iOS is locked down on purpose, and always has been. I wouldn't expect it to change.

Importing photos is one task that can be done, but using the computer is difficult. Generally it is accomplished by the import utility you are using on the computer. I've done it for years with Windows, however with Windows 10, it is now very difficult to have the import utility even import the photos, let alone delete them. It is next to impossible. Even making attempts with Windows File Manager is hit and miss. The tech you spoke with at Apple was mistaken about iTunes and file management. iTunes allows you to sync certain data, such as photos from the computer to the iPhone, sync music from your iTunes library, but there is no access to the file management system on the iPhone. There are a few exceptions now with the Files app on the phone, however the apps that allow that are listed in files. It is not a complete access to the file hierarchy. The iPhone, while it has a camera, is not like the digital camera you replaced with the phone. Working with that camera storage is just that, the storage, and you cannot just work with the iPhone as a storage device.

There is no user file management capabilities on the iPhone. To use it in the manner you wish would require changes that we cannot discuss here.

The reason it lacks such a "basic capability" is why an iPhone has never been hacked (except once by the FBI, and it cost them roughly $1 million). Files are owned by the app that created them. Files are not accessible to any other app than the the app that created them. And they are not accessible to any external user either.

iTunes is not supported on Linux, but it is NOT BEING PHASED OUT. I don't know where that myth came from, but it is untrue, or as they say today (regrettably) FAKE NEWS! . iTunes is being split into 3 separate apps; Music, Podcasts and TV. No functionality will be lost, just moved. As a Linux/Unix user you should appreciate this, as it has been the Unix philosophy since it was created in the early 1970's. An app should do one thing, and do it well. Apps should not include the kitchen sink. iTunes has gotten incredibly bloated over the years since it first appeared in 2000.

iTunes can't do anything with files on your phone other than to back them up. It can sync with apps that you have given it permission to in iTunes configurations. Your camera roll can be read by your computer because it is configured to be a digital camera, so you can copy content off of it the same way you can copy content from any digital camera. An external device does not have write permission to anything on the phone, which is one of the security features. But also keep in mind that iOS is not file oriented, all content is stored in SQLite databases. The photo files are leaf nodes in the Photos database, as is most content that is readable. In addition to the security aspects deleting files will corrupt the database indices unless it is done by the application that owns the database, which can manage updates to the indices. When iTunes updates content on the phone it does it through iOS and the application that owns the content, it does not manipulate files directly. And it can only update content of the built-in apps.