If you want official details, you will need to ask Fandom directly rather than posting here. That said, the high-level answer is that Fandom updated the platform software from its highly-customized yet woefully outdated version to a modern version. Unfortunately, in order to do this, Fandom had to drop most of the customizations users had gotten used to over the years. The button that allows you to add images to a gallery without opening the editor is only one of these customizations that was lost. There are many reasons Fandom dropped so many customizations but here are the largest 3.

1. Simply not compatible with the new software version; sometimes due to new security procedures.

2. Poor documentation of the customization resulting in current staff not knowing how it works and thus lacking the ability to modify it for the new software version.

3. The customization being too server-heavy and Fandom wanting to drop it anyways to cut operational costs.

"It forces us to add images through source editing."

As someone who never uses the visual editor, I don't know how is was back then, but just testing a gallery in ve, it doesn't seem to force me using source editing?

That said...

"it takes more time to add images through source editing"

I fully disagree with this statement.

How can anything take less time than just copy/pasting (or even typing) a filename into the gallery-tags? I would honestly like to know this.

Through visual editor, you can't upload your own images DIRECTLY to the gallery. It asks you to upload an image individually somewhere else and then search the media and post it in the gallery. When the "Add to Gallery" option was still available, this wasn't even an issue. Also if I type the EXACT filename in the source editor it literally just adds an image with a black background and the red link (which I typed) in the middle of it into the gallery, although I follow every step.

@Oot42 btw, just like in the images you sent, it asks you to search for an ALREADY UPLOADED file i.e asks you to add images which already exist on the wiki. When the option was still there, you could directly upload images from your PC to the gallery, even if the image didn't exist on the wiki before.

I know that adding images somewhere else and adding it to the gallery isn't really a problem, but the bureaucrat in the wiki I'm active on considers it vandalism and deletes it.

"you can't upload your own images DIRECTLY to the gallery"

Ok, if that's an issue to you (and was once possible), I see what you mean.

However, you can just use the visual editor's mass uploader and upload everything you want first, and then add it to the gallery the way shown above.

So the statement "it forces us to use source edit" is still wrong. Everything can be done directly in visual editor.

Nevertheless, I still think everything is more time consuming with ve than it is with source editor. Only exception I can think of is editing big and complicated tables. But that's maybe also me not liking visual editing at all, in general, and being used to do everything in source mode only.

Actually, in source editor you have even the possibility to add all uploaded images into a gallery right away, which seems not to be the case in source editor:

"if I type the EXACT filename in the source editor it literally just adds an image with a black background and the red link (which I typed) in the middle of it into the gallery, although I follow every step"

Never seen or experienced anything like this. If you can show me an example of this, I'm happy to look into that issue.

"I know that adding images somewhere else and adding it to the gallery isn't really a problem, but the bureaucrat in the wiki I'm active on considers it vandalism and deletes it."

What?? No matter how and where you upload images, they all end up in the same place, which is the File: namespace. There is zero difference whether you use Special:Upload, the mass uploaders of visual and source editors, or the "add new image" button on Special:NewFiles.

Why would an admin consider something as vandalism depending on the upload method? That doesn't make any sense, honestly.

Hmm, okay, I'll try that. Thank you!