Posting Art with Sources that No Longer Exists

So this is a question that I've ran into from time to time for my searching. Periodically I find a piece of art that I would like to post up on the booru, but the source no longer exists. For example, same-no-kiba's (reblogged from another tumblr:)

https://www.tumblr.com/kira-vera/185824850923

For whatever reason same-no-kiba decided to close their tumblr, but some of their art still exists on other sites. I do like these pieces and feel like they should continue to exist here. This is especially true to art hoarders who saved stuff from who knows how long ago and finding the source may be lost to conventional search engines.

However. the fact that the source no longer does complicates whether or not they are allowed to be so. This further goes into a whole range of complications, such as making sure the artist gets properly cited, whether the art is the best quality, among others.

So I wanted to raise a question about this to see what folks think on how to best proceed when an artist's source no longer

Ah, here's another one:

https://sai-zen-sen.jp/special/takegarou/post-267.html

This is a source that appears to be fanart for a long-running series, but no artist is cited. Great art! But also a bit of a complication when it comes to making sure the proper artist is recognized.

I do it occasionally (and add the relevant "bad_XXXX_id" tag) but generally only for posts which I can confirm via SauceNAO.

As long as you are pretty sure who the artist is (based on artstyle, history etc) it's fine to tag them. source:*bakaookami* is an example of a commissioner where all the works with an artist tag had to be identified by artstyle.

You should definitely try your best to find the original artist, but if that does completely fail, it's better for danbooru to have it without an arttag than not having it at all, so add artist request and maybe post some clues you have in the comments (see source:*bakaookami* artist_request as an example)

For works where you know the artist, but aren't able to give a proper URL for technical reasons, it's fine. In your first example, you would upload using https://www.tumblr.com/kira-vera/185824850923 and use third-party source.
I posted a huge amount of unsourced stuff by obj shep because she did a (free) Gumroad bundle of artwork that is not attributable.
Just remember that if you make a mistake like accidentally uploading a third-party edit or misattributing an artist, it's going to reflect poorly on you.