I'm pretty sure what those websites want is not for you to add your alt text to the metadata of your image file but to give it when your uploading|linking to an image on their platform.
An example on lemmy:
This way users of the social platform that might have a hard time seeing can have a description of the image you uploaded read out to them. Makes the social site more welcoming.
I don't think those social platforms are going to even look at the metadata for anything other than testing if mime type matches dot extension. Although if they are invasively harvesting data they might try to collect location, device type and time data.