Add HTTP3 breaking change documentation for .NET 10 Preview 6

Co-authored-by: gewarren <[email protected]>

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 |
 |-------|-------------------|--------------------|
+| [HTTP3 support in System.Net.Http 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 +1,55 @@
+---
+title: "Breaking change - HTTP3 support in System.Net.Http disabled by default with PublishTrimmed"
+description: "Learn about the breaking change in .NET 10 where HTTP3 support is disabled by default when PublishTrimmed or PublishAot is set to true."
+ms.date: 1/20/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/47443
+---
+
+# HTTP3 support in System.Net.Http disabled by default with PublishTrimmed
+
+Setting `PublishTrimmed` or `PublishAot` to `true` in the project disables HTTP3 support in System.Net.Http by default.
+
+## Version introduced
+
+.NET 10 Preview 6
+
+## Previous behavior
+
+Previously, HTTP3 support was allowed, but might not have actually worked by default in environments where the msquic native library is not available. This resulted in HTTP3 not actually working while the app carried all the code related to it.
+
+## New behavior
+
+Starting in .NET 10, HTTP3 is disabled and the code for it is not 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 HTTP3 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 to reduce application size and complexity.
+
+## Recommended action
+
+To enable HTTP3 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>
+```
+
+Or for AOT scenarios:
+
+```xml
+<PropertyGroup>
+  <PublishAot>true</PublishAot>
+  <Http3Support>true</Http3Support>
+</PropertyGroup>
+```
+
+## Affected APIs
+
+None.

docs/core/compatibility/toc.yml

@@ -62,6 +62,8 @@ items:
                 href: interop/10.0/search-assembly-directory.md
           - name: Networking
             items:
+              - name: HTTP3 support in System.Net.Http 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