Modern chargers regulate the charging speed to prevent overcharging, typically before reaching 100% capacity. This regulation is achieved through communication between the charger and the device (e.g., iPad).
The operating system employs a slight trick to manage charging. When the battery level drops to 99.9% during heavy usage, the system does not send a charging signal. However, it continues to display the battery level as 100%.
The exact limit is not entirely clear for me, but it is likely to be around 97%. This prevents the battery from being fully charged at all times.
Therefore, do not be concerned; always keep it charged.
In essence, I am referring with 100% to the complete utilization of your remaining capacity. The same is true if you set for 80% limit. By the way, iPhone, iPad, and MacBook should still retain 80% of their original capacity after 1000 full cycles. Batteries for solar panels nowadays can endure up to 10,000 cycles.