The tag implication #41232 unavailable_at_source -> source_request has been rejected.

Reason: The wiki page for the former says "Images with broken sources should always have the source request tag until a new source is discovered."

EDIT: The tag implication unavailable_at_source -> source_request (forum #316577) has been rejected by @bitWolfy.

I'm not a fan of this. The wiki page was made by a regular site user, it's not an official policy.

For me, source_request means "I have no idea where this came from, please help me track this down".
On the other hand, unavailable_at_source means "I have the source, but the link is broken". There might not be an alternate source for the image – in that case, the source_request tag wouldn't make much sense.

Source request should just be for images that don't have sources at all imo.