I generally don't add any alt text to images here on SE. But that doesn't mean that I write inaccessible posts. In most cases when I use images, they're for illustration, and if they contain important information that is also present in the main text of the post.
The main goal is to make the post usable even if you can't view the images. Image descriptions aren't sufficient or necessary for that. I also often put a short description of the image directly before it in the main text, so that readers know what they're supposed to look at.
The biggest issue by far is something like a graph with values, or a screenshot of text that isn't repeated in the main text of the post itself.
A naive user getting a prompt to enter an image description might just put something like "graph of X" or "screenshot of error message" in there. But what is actually needed are the actual contents of the image as text, not a description of what is in there. Of course someone with experience and understanding of accessibility won't make that mistake, but that is probably not the group you're targeting with that feature.
I think it is reasonable to point users towards this in the review state. But I would be careful on how to write the guidance for this. It should mostly indicate that you should make sure that the text of your post contains all information, and that you can understand it even without looking at the images (unless that is impossible for the topic).
I also would not make adding the descriptions mandatory (I don't think you're asking for that). Putting the relevant information in the main text also works.