diff --git a/PLATFORMS b/PLATFORMS
index 54c64b6253..aba9e423b9 100644
--- a/PLATFORMS
+++ b/PLATFORMS
@@ -1,9 +1,11 @@
 Supported platforms
 
 In general, this version of BIND will build and run on any POSIX-compliant
-system with a C99-compliant C compiler, BSD-style sockets, and the OpenSSL
-cryptography library. To build with multiprocessing support, a
-C11-compliant C compiler will be needed for standard atomics.
+system with a C99-compliant C compiler, BSD-style sockets with RFC-compliant
+IPv6 support, POSIX-compliant threads, and the OpenSSL cryptography library.
+Atomic operations support from the compiler is needed, either in the form of
+builtin operations, C11 atomics or the Interlocked family of functions on
+Windows.
 
 ISC regularly tests BIND on many operating systems and architectures, but
 lacks the resources to test all of them. Consequently, ISC is only able to
@@ -53,4 +55,6 @@ These are platforms on which BIND is known not to build or run:
 
   * Platforms without at least OpenSSL 1.0.2
   * Windows 10 / x86
-  * Windows Server 2012
+  * Windows Server 2012 and older
+  * Platforms that don't support IPv6 Advanced Socket API (RFC 3542)
+  * Platforms that don't support atomic operations (via compiler or library)
diff --git a/PLATFORMS.md b/PLATFORMS.md
index 4beba7a031..6f2c0deaf6 100644
--- a/PLATFORMS.md
+++ b/PLATFORMS.md
@@ -11,9 +11,11 @@
 ## Supported platforms
 
 In general, this version of BIND will build and run on any POSIX-compliant
-system with a C99-compliant C compiler, BSD-style sockets, and the OpenSSL
-cryptography library. To build with multiprocessing support, a
-C11-compliant C compiler will be needed for standard atomics.
+system with a C99-compliant C compiler, BSD-style sockets with RFC-compliant
+IPv6 support, POSIX-compliant threads, and the OpenSSL cryptography library.
+Atomic operations support from the compiler is needed, either in the form of
+builtin operations, C11 atomics or the Interlocked family of functions on
+Windows.
 
 ISC regularly tests BIND on many operating systems and architectures, but
 lacks the resources to test all of them. Consequently, ISC is only able to
@@ -61,4 +63,6 @@ These are platforms on which BIND is known *not* to build or run:
 
 * Platforms without at least OpenSSL 1.0.2
 * Windows 10 / x86
-* Windows Server 2012
+* Windows Server 2012 and older
+* Platforms that don't support IPv6 Advanced Socket API (RFC 3542)
+* Platforms that don't support atomic operations (via compiler or library)
diff --git a/doc/arm/notes.xml b/doc/arm/notes.xml
index f326885798..675e24a408 100644
--- a/doc/arm/notes.xml
+++ b/doc/arm/notes.xml
@@ -51,6 +51,38 @@
     </para>
   </section>
 
+  <section xml:id="relnotes_platforms"><info><title>Supported Platforms</title></info>
+    <para>
+      BIND 9.13 has undergone substantial code refactoring and cleanup,
+      and some very old code has been removed that was needed to support
+      legacy platforms which are no longer supported by their vendors
+      and for which ISC is no longer able to perform quality assurance
+      testing.  Specifically, workarounds for old versions of UnixWare,
+      BSD/OS, AIX, Tru64, SunOS, TruCluster and IRIX have been removed.
+      On UNIX-like systems, BIND now requires support for POSIX.1c
+      threads (IEEE Std 1003.1c-1995), the Advanced Sockets API for
+      IPv6 (RFC 3542), and standard atomic operations provided by the
+      C compiler.
+    </para>
+    <para>
+      More information can be found in the <filename>PLATFORM.md</filename>
+      file that is included in the source distribution of BIND 9.  If your
+      platform compiler and system libraries provide the above features,
+      BIND 9 should compile and run. If that isn't the case, the BIND
+      development team will generally accept patches that add support
+      for systems that are still supported by their respective vendors.
+    </para>
+    <para>
+      As of BIND 9.13, the BIND development team has also made cryptography
+      (i.e., TSIG and DNSSEC) an integral part of the DNS server.  The
+      OpenSSL cryptography library must be available for the target
+      platform.  A PKCS#11 provider can be used instead for Public Key
+      cryptography (i.e., DNSSEC signing and validation), but OpenSSL is
+      still required for general cryptography operations such as hashing
+      and random number generation.
+    </para>
+  </section>
+
   <section xml:id="relnotes_download"><info><title>Download</title></info>
     <para>
       The latest versions of BIND 9 software can always be found at
@@ -349,7 +381,7 @@
       <listitem>
 	<para>
 	  BIND can no longer be built without DNSSEC support. A cryptography
-	  provder (i.e., OpenSSL or a hardware service module with
+	  provider (i.e., OpenSSL or a hardware service module with
 	  PKCS#11 support) must be available. [GL #244]
 	</para>
       </listitem>