Update Microsoft.Featuremanagement.md (#1090)

* Update Microsoft.Featuremanagement.md

* Update Microsoft.Featuremanagement.md

* Update releaseNotes/Microsoft.Featuremanagement.md

Co-authored-by: Jimmy Campbell <jimmyca@microsoft.com>

---------

Co-authored-by: Jimmy Campbell <jimmyca@microsoft.com>

Author: zhiyuanliang-ms

releaseNotes/Microsoft.Featuremanagement.md

@@ -2,7 +2,9 @@
 
 [Source code][source_code] | [Package (NuGet)][package] | [Samples][samples] | [Product documentation][docs]
 
-## 4.2.1 - July 9, 2025
+## 4.2.1 - July 9, 2025 (Delisted)
+
+This release has been delisted due to the enhancement below resulting in an unintended breaking change. For more information, please go to [#550](https://github.com/microsoft/FeatureManagement-Dotnet/issues/550).
 
 ### Enhancements