With iOS 13 Apple gave users the ability to use Memoji on their device even without a TrueDepth camera, unfortunately not everyone likes this feature and removing this is easy. To do so, follow these steps:
Click into a random conversation thread
Swipe on the app icons until you see the bubble with three dots and then tap it
Deselect “Memoji” from both the “FAVORITES” and the “MORE APPS” section
Going forward, when you select the emoji option on your iMessage keyboard you will no longer be presented with Memoji stickers by default.