It seems that the Prefect 3.x documentation doesn't provide specific information about customizing the arrows in the flow run charts. Additionally, there are no related GitHub issues regarding the ability to bring back arrows in flow run charts within the Prefect repository.
If this feature is critical for your visualization needs, I recommend raising a feature request on the Prefect GitHub repository. This way, the Prefect development team can consider adding or re-implementing this feature in future updates.
You can create a new issue
here.
For now, you may want to explore other visualization options that Prefect 3.x offers or consider reaching out to the Prefect community for potential workarounds.