Nitpicker, the Golem legend is way older than Reb Yiddel Rosenberg. The auther of Frankenstine said she got the idea from the Golem story. It is famously mentioned in the Bnei Yisaschar that the Maharal used the Sefer Yetzira. There are a few other stories about the Golem.
One of the famous “proofs” that he didn’t (usually a great undertaking) make one is that “his Talmid Reb Dovid Ganz” doesn’t mention it. I don’t know who ever made him a close Talmid, if there were any. Does he ever quote a Pshat from the Maharal? Even the Tosafos Yom Tov, who is often called a Talmid of the Maharal, doesn’t quote anything from the Maharal that is not from the Sefarim.
Second of all, his entry on the Maharal is in the Jewish year 252, when the Maharal came back to Prague to be the Rav.
True, we can’t trace the story back. Some say that perhaps the Golem of Reb Eliyahu Baal Shem, which we know about, was confused with the Maharal, since Reb Eliyahu Baal Shem also lived in Prague eventualy.
Perhaps it was only known to the non-Jews, who had personal experience.