When a refurbisher replaces a used battery with a brand new one in the latest iPhones (XR, XS, XS Max and later generations), an "Important Battery Message" appears:
- Settings > Battery, next to Battery Health.
Indeed only an Apple Genius Bar or an Apple Authorized Service Provider can replace a battery in latest iPhones without getting this message. This does not mean that your battery has an issue, only that it was replaced by a new battery that can work as efficiently as the original one.
Also, getting this message doesn't throttle the iPhone's performance. But you won't be able to easily see your battery's health and know when it's time to replace it.
Fortunately, your replacement battery will continue to work perfectly fine, and you'll get all of the benefits that come with a new battery.
If you want to learn more about it.