[eluser]xwero[/eluser]
I agree there is a possibility a lot of data will be retained, but you have to decide how accurate you want to make your read/unread status per user.
For the notify i again would go for a general user setting to let the user choose if none ,all new posts, a digest of posts or individual posts need notification. If the individual posts setting is chosen another table comes into play where you store the thread ids, post ids aren't needed here as you mentioned yourself.
I wouldn't put it in the read table because then you have the chance the notification setting will be removed if the table is cleaned for the user for some reason. You can set an expiration date on the read records but as long as the thread is open it can be revived.
Basically you have to encourage users to clean up after themselves if you want the most accurate data per users in balance with a good performance of the forum. You can do it for example by pointing out similar topics the user is notified of when an new individual thread is selected. If a user has read items older than a certain date you can show a message with a link that will remove those.
