To answer the question in the title,
Is the purpose of allowing practically anyone to edit a question/answer to allow material changes to the original author's intent?
Generally, no. It might even be considered as vandalizing if it worsens the post.
However, I don't see this edit worsens the question.
Now to address this specific case, let's focus on the question and its edit first. (Note: this is my opinion as a 3rd-party who read the post as the first time reader)
In addition to adding minor context (camera model) and removing fluff ("please ask for clarification [...]", the editor removed the dead link and 5 additional words ("If you download it and"), then reworded them to improve the flow.
I don't see any problem with the removal of the dead link; it couldn't be salvaged either, so it won't be useful to anyone as of current writing. From the edit comment, I could see the editor did try to salvage the photos by trying archive.org, but there's none.
Now, probably adding a statement "my Flickr account" would preserve the context, but I don't really see if it helps the question. Personally, I see the question itself is already okay as a general reference, without Flickr and/or sample photos.
However... looking at the overall situation, apparently some of the answers are referring to the specific photos. This is an unfortunate situation where the source photos aren't available anymore but the answers depend on them, making them not really useful. (Really, I'm not sure I understand most of this answer's "looks fine" or "looks okay", or this answer's "this image").
On the other hand, this answer looks the best since it doesn't depend on any of the sample photos at all, making it a suitable answer for a general reference.
Other answers have some insight that can be salvaged by:
- removing the reference to the sample photos, and/or
- explaining the problematic area more generally, e.g.
On the shot with the sunglasses, you appear to be within the minimum focus distance and so the lens is unable to achieve focus. -- this answer
If you're shooting within the minimum focus, the lens won't be able to achieve the focus.
So, it boils down to these questions
But absent of that possibility, do we really want to throw the baby out with the bathwater here? Particularly with a question that is over four years old and has become one of the community's "standard" questions?
Personally, I don't really see "the bathwater" (perhaps you could clarify it on the question/comment?). The question as-is looks okay as a general reference, even more for "a question that is over four years old and has become one of the community's "standard" questions".
I really couldn't blame the editor. If the community want to consider that question as a canonical/general reference, then I'd comment and request the answerers to revise and improve their answers and make them more general instead.