Which tab do you mean? ‘Suggested’ does seem to be all new/unread*, but ‘Related’ definitely includes topics I’ve read.
*EDIT: ‘Suggested’ shows me all new/unread here on Meta – since there’s years of stuff I haven’t read. But the Suggested list – aka “New & Unread Topics” where Related is not active – can get padded with old/read topics: New & Unread Topics can be random topics.