There's an automatic archive system in place for threads that exceed a certain number of posts, it can be found by clicking the [Archive]
link above the top thread and then clicking the [Thread Archive]
link that appears under the title. It's currently broken in that it generates an incorrect URL when accessing a thread older than a few months, but changing "featured" to "archive" in the URL should work for these threads.
Some images and even whole threads might be missing here and there as we underwent a few software and host changes over the years.
Threads on the non-listed boards are not archived fully and instead are deleted after 30 days. Occasionally a staff member will manually archive a thread from there to preserve fun times. If you'd like us to do that more often you can just propose so in the thread itself. I'll personally be reluctant to archive threads based around depression or focused on the OP's personal trauma or experiences, as I believe most users posting about their struggles or sadness would not want those posts immortalized.