> I don't know why everyone seems to think that HTTP redirects are visible in Referer
They would be if it's a same-origin redirect, no? And I was under the impression that 3xx also set it cross origin (barring a referrer-policy header), though I'm less confident now. (I can't test it ATM).
Edit: I am clearly confused. The browser preserves the original referer when performing a 3xx, as you said.