It's not like the alt attribute stopped existing or anything. This simply lets you make what amounts to pretty screencaps of your code
And when you actually have control over the content you can use some nice CSS to get the same look with a fraction of the size and 100% accessible.
Facebook: https://www.facebook.com/help/214124458607871
Instagram: https://help.instagram.com/503708446705527
It's becoming a lot more common!