There are some definitions at the following URLs that could be useful:
danbooru.donmai.us/wiki_pages/5650 - Danbooru's tag wiki (obviously)
redpill.booru.org/index.php?page=forum&s=list - redpill's tag wiki (e.g., this place has no page for "nazi_salute")

1) What makes redpill a reputeable repository or relevent to Eastern animations?

2) There's no need for a nazi_salute tag. Just searching nazi and salute will find exactly what you want.

Secondly, the "Nazi salute" isn't Nazi at all, let alone German. Supposedly, though this is dubious, the Romans used the salute in their time, and hence it was adopted by the Italian fascists and the NSDAP in turn copied the salute from the Italians. Calling it a Nazi salute would be inaccurate since it was a fascist salute in general use in the 20th century.

Very little of the information from that page actually applies or is useful on Gelbooru. Most of the stuff that is useful is covered elsewhere (help pages, gardener's guild thread), and what remains from that would leave a very empty and dull entry.