It heavily depends on the usage pattern. If you charge in the evening to 100%, let it on the charger overnight and then during the day discharge to 0%, that might kill the battery completely within 2 years.
Keeping it within 20-80% most of the time might keep the battery healthier.
20 is a bit low - I would stay above 40.
Why
Because 20% will still harm a battery (less then 0% obviously), above 40 to 80 approximately the no-harm zone.
I bought my iPhone XS at launch and it just reached 80%, although on the latest version of iOS 80% doesn't get you as far as it once did. I'm not a super heavy smartphone user though.
I have a 3 year old phone that's at 85%. That seems to be a pretty normal pattern for me over the past 10+ years.
iPhone 11 Pro Max (Sep 2019) on 86% here.
Same phone, sitting at 82% without caring too much about charging patterns.