A "pristine realm" is a term used by J. Allen Brack in a blue post on April 25, 2016 posted Nethaera that discussed how the issue of a retro or vanilla realms within the company (not speaking to emulated or private servers directly) written by J. Allen Brack.
Here is an excerpt from the blue post that discusses it:
So what can we do to capture that nostalgia of when WoW first launched? Over the years we have talked about a “pristine realm”. In essence that would turn off all leveling acceleration including character transfers, heirloom gear, character boosts, Recruit-A-Friend bonuses, WoW Token, and access to cross realm zones, as well as group finder. We aren’t sure whether this version of a clean slate is something that would appeal to the community and it’s still an open topic of discussion.