Add HTTP3 breaking change documentation for .NET 10 Preview 6 (#47526)

Author: Copilot

docs/core/compatibility/10.0.md

@@ -68,6 +68,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 
 | Title | Type of change    | Introduced version |
 |-------|-------------------|--------------------|
+| [HTTP/3 support disabled by default with PublishTrimmed](networking/10.0/http3-disabled-with-publishtrimmed.md) | Source incompatible | Preview 6 |
 | [HttpClient/SslStream default certificate revocation check mode changed to Online](networking/10.0/ssl-certificate-revocation-check-default.md) | Behavioral change | Preview 6 |
 | [Streaming HTTP responses enabled by default in browser HTTP clients](networking/10.0/default-http-streaming.md) | Behavioral change | Preview 3 |
 

docs/core/compatibility/networking/10.0/http3-disabled-with-publishtrimmed.md

@@ -0,0 +1,51 @@
+---
+title: "Breaking change - HTTP/3 support in System.Net.Http disabled by default with PublishTrimmed"
+description: "Learn about the breaking change in .NET 10 where HTTP/3 support is disabled by default when PublishTrimmed or PublishAot is set to true."
+ms.date: 07/22/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/47443
+---
+
+# HTTP/3 support in System.Net.Http disabled by default with PublishTrimmed
+
+Setting `PublishTrimmed` or `PublishAot` to `true` in the project disables HTTP/3 support in <xref:System.Net.Http> by default.
+
+## Version introduced
+
+.NET 10 Preview 6
+
+## Previous behavior
+
+Previously, HTTP/3 support was allowed, but might not have actually worked by default in environments where the msquic native library wasn't available. This resulted in HTTP/3 not actually working while the app carried all the code related to it.
+
+## New behavior
+
+Starting in .NET 10, HTTP/3 is disabled and the code for it isn't included when `PublishTrimmed` or `PublishAot` is set to `true`.
+
+## Type of breaking change
+
+This change can affect [source compatibility](../../categories.md#source-compatibility).
+
+## Reason for change
+
+Since making HTTP/3 work requires extra gestures and often doesn't work by default, the runtime no longer carries the code for it in trimmed or AOT-compiled applications.
+
+## Recommended action
+
+To enable HTTP/3 support in applications that use `PublishTrimmed` or `PublishAot`, set the `<Http3Support>` property to `true` in your project file:
+
+```xml
+<PropertyGroup>
+  ...
+  <PublishTrimmed>true</PublishTrimmed>
+  <Http3Support>true</Http3Support>
+</PropertyGroup>
+```
+
+## Affected APIs
+
+None.
+
+## See also
+
+- [Trimming options](../../../deploying/trimming/trimming-options.md)

docs/core/compatibility/toc.yml

@@ -62,6 +62,8 @@ items:
                 href: interop/10.0/search-assembly-directory.md
           - name: Networking
             items:
+              - name: HTTP/3 support disabled by default with PublishTrimmed
+                href: networking/10.0/http3-disabled-with-publishtrimmed.md
               - name: HttpClient/SslStream default certificate revocation check mode changed to Online
                 href: networking/10.0/ssl-certificate-revocation-check-default.md
               - name: Streaming HTTP responses enabled by default in browser HTTP clients

docs/core/deploying/trimming/trimming-options.md

@@ -64,10 +64,10 @@ The SDK also makes it possible to disable debugger support using the property `D
 
 Several feature areas of the framework libraries come with trimmer directives that make it possible to remove the code for disabled features.
 
-| MSBuild property | Description |
-| - | - |
+| MSBuild property         | Description |
+|--------------------------|-------------|
 | `AutoreleasePoolSupport` | When set to `false`, removes code that creates [autorelease pools](../../runtime-config/threading.md#autoreleasepool-for-managed-threads) on supported platforms. `false` is the default for the .NET SDK. |
-| `DebuggerSupport` | When set to `false`, removes code that enables better debugging experiences. This setting also [removes symbols](#remove-symbols). |
+| `DebuggerSupport`        | When set to `false`, removes code that enables better debugging experiences. This setting also [removes symbols](#remove-symbols). |
 | `EnableUnsafeBinaryFormatterSerialization` | When set to `false`, removes BinaryFormatter serialization support. For more information, see [BinaryFormatter serialization methods are obsolete](../../compatibility/serialization/5.0/binaryformatter-serialization-obsolete.md) and [In-box BinaryFormatter implementation removed and always throws](../../compatibility/serialization/9.0/binaryformatter-removal.md). |
 | `EnableUnsafeUTF7Encoding` | When set to `false`, removes insecure UTF-7 encoding code. For more information, see [UTF-7 code paths are obsolete](../../compatibility/core-libraries/5.0/utf-7-code-paths-obsolete.md). |
 | `EventSourceSupport` | When set to `false`, removes EventSource-related code and logic. |