The idea of a previous age(s) of magic is actually pretty well publicized in Shadowrun at this point.
Ehran the Scribe discusses the cycles of Magic back in 1st edition and gave a publicized speech about it.
The Atlantean Foundation is built on the idea of discovering "Atlantis" (Which is Thera) and digging up information and artifacts about the 4th world. They have 20+ Million member worldwide and a published magazine.
The Artifact Rush in Seattle Missions wasn't particularly secretive. Keep in mind it was capped off (Or was supposed to be capped off, at least, till they get stolen again) with a public viewing of all the artifacts collected during the season.
The idea of a previous age, a previous World of Magic (or even multiple ages) isn't a new one in Shadowrun and is pretty widespread. The specific details, though, are pretty muddy, clouded in half-truths from teh few who actually know it, lies, rumors, speculation, etc.