But what else can be done?
With imgur? Out of the options given in the OP, I would still probably lean towards #3 or #4.
These allow the best view for threads for any images still hosted at imgur and would still allow the image proxy to put up the "invalid image"
image for any deleted images, and not a broken link.
People using TOR are usually the ones concerned with their IP privacy, so why does an exemption here matter to them if they're IP is already being covered by Tor? Imgur would only see the Tor node's IP.
For people not using TOR, what would be the main concerns with
imgur knowing your IP address viewed an image on the forum?
For #5, this would certainly deal with the deleted imgur images nicely. Are there maybe trust/liability issues here when theymos picks a specific place to force redirect and/or edit the posts of other users image(s) to a particular host? Options in the OP are just sort of using what the user posted, but displaying them differently.
For the future?
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I had mentioned (privately) the thought of the forum itself hosting/archiving sets of images from particular domains through the image proxy, potentially for future situations like this. Hosting sets of images on the forum itself is probably the best case scenario for historical preservation purposes, but based on the options presented in the OP, I don't think this is really something theymos is willing to entertain, probably for good reasons.
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Essentially that the forum would save a backup copy of images as they are seen through the image proxy. Not for [immediate] display purposes, but for archiving, so that if/when an image host/domain goes offline, the forum could then display its archived copy of those images.
It may be too complicated to create, and would overall obviously take some more resources in terms of storage space to save an image the first time it is seen, and bandwidth to display any images that are using the archived copy, aaand probably some slight increases in things like processor/memory/DB activity to manage it all.
Maybe overkill here but I was thinking more long-term, or in situations where users or the community may not get a chance to save the images. I believe images are a big part of the forum, so losing them and/or access to them in large batches seems bad for the forum.