-bytes long--that is, this slice syntax does not change the bytes
-returned at all, but can be used to provide information about the
-underlying object store.
+bytes long. Older versions of Cumulus can also use the syntax
+ [=<length>]
+as a synonym for length assertions, but this notation is deprecated.
+
+(In older versions of the format, the syntax [<length>] was a shorthand
+for [0+<length>]: that is, select the first <length> bytes of the object
+but make no assertions about the overall size. The backup tool has not
+generated such slices since v0.8.)