The reliable answer is that using a third-party fast charge without MFi certification to charge the iPhone 11, the battery health will certainly decline quickly. There is an MFi certified charger, it will not affect.
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Compared to other phones, the iPhone battery is very delicate. If the charger is not matched, the health of the iPhone battery will decline very quickly. The author's iPhone 8 Plus used the USB port on the socket to charge the phone, and the battery health decreased by 5% in a week, and the battery health decreased by 1% in two months after replacing the original 5W charger.
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Later, I specially did a test and bought a third-party brand charging head, which has Apple MFi certification and supports 18W fast charge. Using this third-party charger to charge the iPhone 8 Plus, the battery health also dropped 1% in 2 months, as Apple's original 5W charger.
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By comparison, it was found that charging an Apple phone with a charger without MFi certification would cause the battery health to decline quickly. Obviously, Apple mobile phones can use third-party brands of chargers, must have MFi certification.