Scenario in short:
I’m trying to install LineageOS onto a spare phone that I have. Hoping to get a daily driver out of it. The installation process requires me to install Android11 to install lineage on top of /over that. Which afaik isn’t directly available outside of some websites that I’ve never heard of.
For safety’s sake: would the install of lineage wipe the install of Android11? Or could there still remain backdoors?
Using an older OS is only a security issue if you actually do stuff with it. If you’re downgrading just to reinstall a newer OS then what malware is going to exploit your phone in that short period of time?
As others have said the real issue will probably be running old firmware for other things like your modem. But even then someone still has to be looking to explicitly exploit that.