The folks at XDA Developers spotted the feature in a teardown of the Digital Wellbeing app back in September. A beta version now appears to be rolling out, with Twitter user @jay__kamat receiving it on his Pixel 4A handset.
Once you enable Heads Up—found inside the ‘Reduce Interruptions’ section of the Digital Wellbeing app—and allow location (optional) and physical activity permissions, you’ll be warned whenever movement is detected while the screen is active. Alerts include “Be careful” and “Look ahead,” though Google does note: “Use with caution. Heads Up doesn’t replace paying attention.”