So I was looking into an artist, not gonna say who for privacy reasons, and they have deleted their NSFW Tumblr as well as their FurAffinity account, plus some of their NSFW images on FA, meaning links to them are dead. I managed to find the Tumblr account on archive.org, and it seems to have most of their NSFW images that are posted here and then some. My question is, would it be okay to link that as a source, as well as upload images from there that aren't already on e6? Or would it be better to clear the dead-link sources on the images that exist here and not upload any more of their images? I'm thinking the latter to respect the artist's possible wishes, but I want some second opinions.

Make sure that:
1. The artist is not on the avoid_posting list
2. They did not put their work behind a paywall somewhere
3. The artwork meets the uploading_guidelines

If that is the case, feel free to upload those images. E621 is an art archive, after all, and we would prefer those posts to not be lost.
While we do encourage all uploaders to get permission from the artist, it is not a requirement.

Additionally, you should run images through the similar image search to make sure they were not uploaded already.
It's possible that the artist has taken down some of their work from e621 as well. If that's the case, those posts will show up in that search, and you should not re-upload them.

5h3i1ah said:
My question is, would it be okay to link that as a source, as well as upload images from there that aren't already on e6?.

Archive.org would be a valid source, you will just have to add in the archived_source tag in the post.

Or would it be better to clear the dead-link sources on the images that exist here and not upload any more of their images?

Clearing dead-link source links from existing posts is NOT recommended, as people can still attempt to find them through archive sites like you did.
Add in the unavailable_at_source tag for posts that have dead-link sources.

I'm thinking the latter to respect the artist's possible wishes, but I want some second opinions.

You should always ask the artist of their wishes directly, not by asking random third-parties.
If they had deleted their NSFW artworks from their galleries, it is most likely that they do not wish the public (or potential employers) to see their NSFW artworks anymore.

While not mandatory, I would highly advise you to obtain permission from the artist to (1) repost their artworks on e621.net, and (2) repost their deleted NSFW artworks as well.
If they are not consulted beforehand, you run the risk of the artist getting offended and taking down/nuking all of their artworks from this site and listing themselves as DoNotPost in the future.