iPhone Charging Routine

No, that charging routine is not true, and it won’t harm the battery health to charge to 100%. Leaving it at 100% for long periods of time is not good for the battery, but as it always uses energy it won’t stay at 100% for long after you remove it from charging.

The absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates. Here's more information→About Optimized Battery Charging on your iPhone - Apple Support

Simple guidelines if you want to preserve the life of your battery:

Enable Optimized Battery Charging on your phone.

Plug the phone into the charger and leave it connected all night....every night