Add non-normative note for event-level reports deletion for unexpired…

… destination limit (WICG#1351)

Co-authored-by: Andrew Paseltiner <[email protected]>

index.bs

@@ -2890,6 +2890,14 @@ To <dfn>delete sources for unexpired destination limit</dfn> given a [=set=] of
         and |report|'s [=event-level report/trigger time=] is greater than or equal to |now|:
         1. [=set/Append=] |report|'s [=event-level report/report ID=] to |deletedEventLevelReports|.
         1. [=set/Remove=] |report| from the [=event-level report cache=].
+
+    Note: Leaking browsing history of destinations deactivated for unexpired
+    destination limit from [=event-level reports=] whose [=event-level report/trigger time=]
+    is earlier than |now| is mitigated by the presence of [=obtain a fake report|fake reports=].
+    [=Event-level reports=] whose [=event-level report/trigger time=] is greater
+    than or equal to |now| must be deleted to avoid exposing whether an
+    [=attribution source=] has a [=attribution source/randomized response=].
+
 1. Let |deletedAggregatableReports| be a new [=set=].
 1. [=set/iterate|For each=] [=aggregatable attribution report=] |report| of the [=aggregatable attribution report cache=]:
     1. If |sourcesToDelete| [=set/contains=] |report|'s [=aggregatable attribution report/source identifier=]: