I remember doing this on Reddit Sync years ago by combining this option,

Settings shortcut: History > Mark posts as read on scroll

With something like this option,

Settings shortcut: General > FAB action

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.