I mean, exactly how invasive are default operating systems? (Like Windows, Mac, Chrome OS, Android, iOS) Do they log your keystrokes, log passwords, capture screen, upload your photos, videos, or audio? (Assuming you aren’t a target of government) Is it even possible for the average person who doesn’t feel comfortable messing with installing operating systems to have any privacy?
You can buy computers and phones with those OSes preinstalled, so it’s not necessarily “custom”. Otherwise, no.