This type of warning has recently started to appear on newer Apple models when the battery or another component has not been replaced by Apple itself.
However, you don't need to worry. These messages simply mean that a component on your device has been replaced by our reseller with an equivalent replacement part and professionally installed so that you can use your refurbed iPhone for as long as possible. After 4 days, the message disappears from the lock screen, after that it is only visible in the settings and has no negative impact on the performance or battery capacity of your device.