I can't find proof that this guideline was adopted as an Official True™ Guideline through the WP:PROPOSAL process. Does that mean it's not legitimate?
The WP:PROPOSAL process itself was adopted in late 2008. It was based on the processes used for Wikipedia:Identifying reliable sources (medicine) and Wikipedia:Manual of Style/Medicine-related articles. Pages tagged as guidelines or policies before 2008 generally did not follow any consistent process. The older the page is, the more likely it was marked as a guideline or policy as a result of off-wiki discussions (especially in 2003 or earlier) or through a normal discussion on its talk page.

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