I think this would be up to the Discourse people. One of the frustrations with accessibility is that often features get lost when sites get updates. For example, the user icon in the upper right-hand corner is inaccessible using iOS app, although it’s accessible using the computer. I am not really sure where to send these sorts of issues, and also at times get tired of feeling like I’m complaining all the time, so I sometimes just put up with them.
Using alt text in this forum used to just use regular HTML code, which is probably one of the things Google may have come up with for you. Then Discourse changed this to make it more obscure. My memory is that when you upload a picture, you can replace anything before the | character with whatever descriptive text you’d like. If it’s text, you could just type what the text says. If it’s something like a CGM screenshot, you could describe what it is, like, “Six hour blood glucose trace flatlined at 14 mmol/L” (or whatever). Usually it’s good to keep it short and, in a context like social media, focus on the features salient to the post (which can be hard with CGM graphs, I know!).
Search engines have definitely co-opted alt text to use for their own purposes beyond accessibility. I’m not sure whether this is a good or a bad thing, but it certainly makes searching for the non-expert interested in accessibility more difficult.
It would be great if the image upload process included a text box that asked for alt text! These types of features are now included in Word, Google Docs, Facebook, Twitter, Instagram…since social media generates so many pictures, it’s now up to users as well as programmers to make their content accessible. Although at the moment, 99.9% of people don’t do it, likely for a variety of reasons. (I recently posted a link on how to add alt text to Facebook images in a community that is 90% image posts and my post was ignored. Didn’t receive a single comment or like in a group of over 5,000 members.) So I do really appreciate people here taking an interest in something that doesn’t directly affect them.
Here’s the thread where it’s come up in the past: