doc(tubes): note that OTIS mock collection perm

tubes/templates/tubes/tube_list.html

@@ -34,6 +34,28 @@ <h3>Proposing</h3>
       Please use a <strong>descriptive title</strong> that doesn't
       spoil the problem but gives an idea for what the problem is about.
     </li>
+    <li>
+      Note that the database rolls over from year to year.
+      If you ever want to withdraw a problem from the database,
+      you need to manually ask OTIS staff to have it withdrawn.
+      Otherwise you should assume by default it can be seen by future years
+      of OTIS as well and possibly appear in subsequent years' mock AIME's.
+      <ul>
+        <li>
+          In practice, we probably won't go out of our way to dig out
+          ancient problems for use, but the point is that
+          right now we don't systematically erase old problems.
+        </li>
+        <li>
+          Even then, because OTIS has such a large community of students,
+          problems submitted to OTIS should no longer be considered secure,
+          even if you ask for the problem to be withdrawn later.
+          If you think you might want to submit a problem to a different
+          competition later in life where problem security is at all relevant,
+          you should not submit it here.
+        </li>
+      </ul>
+    </li>
   </ul>
   <h3>Solving</h3>
   <p>