So I posted some images by the artist 2n5 which I found on e621, and when I was told saucenao sourced them back to deleted pixiv pages, I had a check - some of them in question are sourced to 2n5's tumblr, which is now deleted.

Assuming the 'source' link is gone and can't be replaced, must the image go?

Example: https://e621.net/post/show/863474/2n5-animal_humanoid-barefoot-before_and_after-blon

no? it only needs to be deleted if the artist, character owner etc especially requests for takedown.

Nope.

And to answer another unasked question, even if the source is 404, it's still good to keep intact. Especially for searching purposes e.g. if someone tries to google the URL where the image was supposed to be.

Really hate it when I'm trying to find images source and there's nothing to go by anywhere.

Thanks; I understand that sourcing is important but wasn't aware to the degree it was enforced.