TYPO3 Core

My expectation is that it will work as before - maybe I'm too stupid to find it, but I haven't found any corresponding entry in the changelogs or any hint that this functionality is depricated or something like that.
Also it could be that you have mixed values in the image-array (external urls and system-internal urls).

Very simplified example:

<f:variable name="imageArray" value="{0: 'https://forge.typo3.org/themes/TYPO3/images/custom-topheader.jpg', 1: 'FAL-IMAGE-OBJECT'}" />

<f:for each="{imageArray}" as="image">
    <f:image src="{image}" />
</f:for>

Christian Hackl wrote in #note-3:

My expectation is that it will work as before

That this worked before, is/was more of a bug, than a feature.
Unfortunately making something always "work as before" is not achievable, when changing anything within software.

The ImageViewHelper is designed to process FAL images.
This is even more apparent when looking at the <f.uri.image> view helper, which basically can not do anything when being called with a URL.

In the long run (was planned for many versions, but has not ben done yet) these view helpers will only accept FAL images via the image argument.

- maybe I'm too stupid to find it, but I haven't found any corresponding entry in the changelogs or any hint that this functionality is depricated or something like that.

This has been fixed in the related ticket. As said before, this was considered more of a bugfix to straighten the code in the email templates and remove the code in the view helpers, that were only introduced for this case.

Also it could be that you have mixed values in the image-array (external urls and system-internal urls).

Is this a theoretical case or do you have an example where you would need exactly this?
In the extension you referenced, code could be changed to just use the <img> tag, which works across all TYPO3 versions of course.

Anyway, your example would not work anyway for the following reason:

The "src" argument only accepts strings since the inception of this view helper. So you would need an if condition anyway, to pass {image} to the correct argument like so:

<f:variable name="imageArray" value="{0: 'https://forge.typo3.org/themes/TYPO3/images/custom-topheader.jpg', 1: FAL-IMAGE-OBJECT}" />

<f:for each="{imageArray}" as="image">
    <f:if condition="{image -> my:isString()}">
        <f:then>
            <f:image src="{image}" />
        </f:then>
        <f:else>
            <f:image image="{image}" />
        </f:else>
    </f:if>
</f:for>

But then again, what do you think is the key feature that helps you do something that isn't possible by writing like so:

<f:variable name="imageArray" value="{0: 'https://forge.typo3.org/themes/TYPO3/images/custom-topheader.jpg', 1: FAL-IMAGE-OBJECT}" />

<f:for each="{imageArray}" as="image">
    <f:if condition="{image -> my:isString()}">
        <f:then>
            <img src="{image}" />
        </f:then>
        <f:else>
            <f:image image="{image}" />
        </f:else>
    </f:if>
</f:for>

I would argue, that reading the template I proposed makes the intention much clearer.

Did I miss something?