Imgur is a great website for hosting images, but are they keeping those images forever?
If not, for how long?
Stated by Imgur under Imgur help → Support → Images
How long do you keep the images?
Forever! An image is only removed if deletion is requested.
Imgur used to delete uploaded images that haven't got any views in six months: https://wayback.archive.org/web/20141212172016/http://imgur.com/faq#long
It seems that since February 2015, when Imgur Pro service was discontinued, Imgur has preserved all images: https://blog.imgur.com/2015/02/09/imgur-pro-for-everyone/