The history of alt text goes back to 1993. The alt attribute was introduced for HTML 1.2 to be used for text-based browsers, which load faster by not loading most of the graphic content.
This was a big deal when bandwidth connections were super limited.
This is because graphics require more bandwidth to load quickly due to being larger files (compare a .txt file with a .jpg file).
Folks who remember using the internet in the '90s might also have intentionally forgotten "lag" and images that took forever to load if they loaded at all.
Thus, the alt attribute was created to provide alternate access to the relevant content of graphics that users might not see, for whatever reason, so they can still use a site or page.
The alt attribute accepts text input (thus, "alt text") and displays for sighted users in the place of an image.
In 1999, with HTML 4.1, the alt attribute became required for img and area tags.
Even though this is a really old standard at this point, with lots of documented best practice, I personally took many, many web dev classes through 2015 and none of them ever taught me about the alt attribute.
This might go some way to explaining how even people who are professionals at creating websites and web content generally know and understand very little about alt text and other web accessibility (#a11y) best practices.
It tends to be seen as an optional and very limited specialty.
As I say, there's a lot of documentation on best practice for alt text and that documentation is especially very helpful if you're creating a traditional website.
It is not, unfortunately, written for social media (even the social media guide I've seen assumes access to a traditional website).
One thing that can help is understanding the goal of alt text—it's not image description.
I can't find @hannah.the-void.social's exact phrasing, but she says something like the purpose of alt text on social media being to enable people who need alt text to be full participants in the conversation.
I have a favorite guide (webaim.org/techniques/a...) that does a great job at making clear the importance of context in writing alt text, but not all parts of it perfectly translate to social media.
