Sanitizer support is not new in 5.2 after all.

doc/sphinx/whats-new/upgrading-5.2.rst

@@ -278,13 +278,6 @@ Other changes
     it possible for VMOD authors to load their code into a debugger
     after a varnishd crash. See :ref:`ref_param_debug`.
 
-  * The project build tools now facilitate the use of sanitizer flags
-    (``-fsanitize`` for the compiler and ``ld``), for undefined
-    behavior, threads, addresses and memory. See the options
-    ``--enable-ubsan``, ``--enable-tsan``, ``--enable-asan`` and
-    ``--enable-msan`` for the ``configure`` script generated by
-    autoconf.
-
   * *XXX: ...*
 
 * *XXX: other changes in tools and infrastructure in and around