@red61544
I believe there is no such thing as "constructive criticism"; all criticism is, to some extent, destructive.
Although use of the word "criticism" makes this tough, I'd suggest that "feedback" can be offered in a constructive way, at least sometimes and for some authors. For example, "[Character Name] was super interesting and I enjoyed the way you wrote his scenes. I'd like to see more of him," will, in most cases, be perceived as a positive, a "boost," something to feel good about, even if it suggests the story might have been better.
I agree that feedback from readers is different than that from an editor, although perhaps not as much on SOL as for traditionally published authors. When a story is "published" and a reader says something like, "I really was intrigued by the issue of how the coach would react to the MCs decision to participate in another extracurricular activity, and I would like for the story to have explored that more," -- it's feedback for future stories, but, unless the author is going to publish a revised version, it can't help improve the story.
Finally, "constructive" and "destructive" are judgments the author makes about the communication. Some can treat very negatively-worded critiques with no hint of how to make it better as "constructive." Others may be a bit more prone to take even fairly positive feedback as a hit to their confidence and motivation.