The idea that we would A/B test handwritten vs typed to see what would improve retention is focusing on the wrong thing. It's like A/B testing mayo or no mayo on your big mac to see which version is a healthier meal. No part of the school system is optimized for retention. It's common for students to take a biology class in 9th grade and then never study biology again for the rest of their lives. Everyone knows they won't remember any biology by the time they graduate, and no one cares.

We know what increases retention, it's active recall and (spaced) repetition. These are basic principles of cognitive science have been empirically proven many times. Please try to implement that before demanding that teachers do A/B tests over what font to write the homework assignments in.