I am looking for information on a psychic marauder archetype which was on the Archives of Nethys site, but that site is down.
Where can information on psychic archetypes could be found online?
The Psychic Marauder archetype shows up on the Heroes of the Darklands Player Companion book (page 22). It can be bought on paizo.com, and if done so, it will be available for download on their website (My Downloads on top) anytime you want.
The material could possibly be available on the official PRD sometime in the future, though that is unlikely as the archetype is from a player companion book. Paizo does not bother much to put campaign setting specific material online as that wouldn't help on their sales and that material is not as widely used on Pathfinder Society as the hardcover material. But doesn't seem to bother when fan supported websites do so (like the Archives of Nethys).
That said, the official PRD is still behind schedule, the material from Occult Adventures was the last to be updated there, and not much other than errata has been posted in a while. Nor it has been converted to the unofficial SRD (also fan supported) even if under a different name. Though the SRD does not bother much with the player companion line, and focus their updates on the hardcover line. The book was released earlier this year, so I wouldn't expect it to be available there yet.