Apparently, MSVC has atomics, but they're experimental and hidden.

src/syncterm/SyncTERM.vcxproj

@@ -80,6 +80,7 @@
       <AdditionalIncludeDirectories>%(AdditionalIncludeDirectories);..\sbbs3</AdditionalIncludeDirectories>
       <RuntimeLibrary>MultiThreadedDebug</RuntimeLibrary>
       <LanguageStandard_C>stdc17</LanguageStandard_C>
+      <AdditionalOptions>/experimental:c11atomics %(AdditionalOptions)</AdditionalOptions>
     </ClCompile>
     <Link>
       <SubSystem>Windows</SubSystem>
@@ -99,6 +100,7 @@
       <AdditionalIncludeDirectories>%(AdditionalIncludeDirectories);..\sbbs3;</AdditionalIncludeDirectories>
       <RuntimeLibrary>MultiThreadedDLL</RuntimeLibrary>
       <LanguageStandard_C>stdc17</LanguageStandard_C>
+      <AdditionalOptions>/experimental:c11atomics %(AdditionalOptions)</AdditionalOptions>
     </ClCompile>
     <Link>
       <SubSystem>Windows</SubSystem>

src/syncterm/telnets.c

 /* Copyright (C), 2007 by Stephen Hurd */
 
+#ifndef _MSC_VER
 #if defined(__STDC_NO_ATOMICS__)
 #error Support for stdatomic.h is required.
 #endif