net is a namespace alias for boost::asio:

The namespace alias beast::net replaces boost::asio in all code
and documentation.

test/beast/experimental/timeout_service.cpp

@@ -22,7 +22,7 @@ public:
     void
     run() override
     {
-        boost::asio::io_context ctx;
+        net::io_context ctx;
         set_timeout_service_options(ctx,
             std::chrono::seconds(1));
         pass();