I remember doing this on Reddit Sync years ago by combining this option,
Settings shortcut: History > Mark posts as read on scrollย
With something like this option,ย ย
ย ย
Settings shortcut: General > FAB actionย ย
Set the FAB to hide read posts. The main difference to how I had it working in the past is that with this, you must use the floating action button to periodically purge this content, where as I'm sure I had it working automatically before.