EDIT 2 November 24th: A few days ago, Discord implemented initial alt-text support for image uploads! The feature is still a work-in-progress, but I think it's fair to say that alt-text is no longer behind a paywall.
EDIT 1 July 1st: Discord developer jhgg replied to my Issue on GitHub, noting:
Additional support for alt text within our client is planned for Q3 this year. No further updates at this time.
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Discord has locked an important accessibility feature behind their Nitro subscription. Alt-text, the internet's standard for embedding image descriptions, is only available via the new Custom Stickers feature.
Discord's help page for Custom Stickers details the importance of alt-text:
We strongly recommend adding a short, concise description (alt-text) for each custom sticker. This will allow people who use screen readers to know what your sticker looks like without needing to see the actual sticker.
The only other way to specify image descriptions on Discord is to include description text beside each image, which is far clunkier for screen readers to parse.
Discord's Custom Stickers...
In order of implementation scope, from least to most difficult:
Discord fetches embed data from the Open Graph tags located on webpages.
Discord's embed system supports the og:image tag, which means that posting a non-image link can still embed the relevant image. Expected behavior!
However, this embedded copy of the image omits any of the alt-text information found within og:image:alt.
I've detailed specifics on Discord's issue tracker.
Currently, there's no way for bots to assist users in properly users in adding alt-text to their images.
My partner and I wrote AltTextBot as a best attempt in this direction, but we're limited by the specifics of Discord's API. I've confirmed with other Discord API folks (and with a couple Discord developers) that this is indeed the case.
If you're familiar with how social platforms such as Twitter allow you to include alt-text, this implementation would behave similarly. It would, however, require the most work on the part of Discord's development team.
Alt-text support on Discord is a highly sought-after feature, and has been for years. It's rough to see alt-text relegated to a feature so stringently paywalled :(