Ignoring a DMCA takedown isn't illegal, you just potentially lose the "Safe Harbor" protection if someone does decide to sue you.

It's not different. As in: both are fine. It's not a crime to link to a site that links to content; TPB itself hosts no content. Linking is not a crime.

https://thepiratebay.org

> How is this any different than posting a piratebay URL for a pirated movie on HN?

It's a link to a site that links to the front page of SciHub, not a particular paper accessed through it.

There's a difference between the page for a pirated movie and the front page of TPB.

Where things really get interesting is info hashes. I could right now post a 160 bit SHA1 hash of any content on the bittorrent DHT and you could obtain it simply by tacking "magnet:?xt=urn:btih:" on the front. So perhaps simply sharing the hash of pirated content with some sort of encouragement could be enough to land you in hot water.