The content filter to hide message which have been read is implemented in the next release (v0.56.2).
It is a simple filter that hides whole threads which has been read. Not individual messages.
I hope this is what you have been looking for.
v0.56.2 will be released this week.
Please note that you can also remove all articles in a subscription (read or not) using the "Delete All" option in the context menu. Just in case you haven't noticed that feature 😀