Tested 3309016-add-image-preload branch using DrupalPod in core version 10.1.x
Viewed UI changes through the image field present on the Article content type (Structure → Content types → Article → Manage display).
The UI here looks great; Expanding the image loading dropdown in format reveals the "Preload" checkbox above the Image loading attribute radio. Hiding the attributes section upon checking "Preload" clearly indicates the other attribute options are rejected.
Compared page source before and after updating the image field format with preload active to verify that the respective link tags were placed in HEAD with rel="preload" as="image". Everything appears as described.
Working great! Very useful enhancement to the image field formatter.