With https://github.com/dotnet/runtime/pull/106497/files,

object.GetType is no longer an internalcall. This means we no longer
mark the ctor of the return type, System.Type, so the type check
removal optimization is kicking in where it didn't before.

Author: sbomer

src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/NullableAnnotations.cs

@@ -65,6 +65,9 @@ public static void Main ()
 			GetUnderlyingTypeOnNonNullableKnownType.Test ();
 			MakeGenericTypeWithUnknownValue (new object[2] { 1, 2 });
 			MakeGenericTypeWithKnowAndUnknownArray ();
+
+			// Prevents optimizing away 'as Type' conversion.
+			PreserveSystemType ();
 		}
 
 		[Kept]
@@ -331,5 +334,11 @@ static void MakeGenericTypeWithKnowAndUnknownArray (Type[] unknownTypes = null,
 			nullable.GetProperties ();   // This works - we still know it's Nullable<>, so we can get its properties
 			Nullable.GetUnderlyingType (nullable).GetFields (); // This must warn - since we have no idea what the underlying type is for the unknownTypes case
 		}
+
+		[Kept]
+		static void PreserveSystemType ()
+		{
+			typeof (Type).RequiresNonPublicConstructors ();
+		}
 	}
 }

src/tools/illink/test/Mono.Linker.Tests.Cases/UnreachableBlock/ComplexConditions.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Reflection.Emit;
 using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Helpers;
 using Mono.Linker.Tests.Cases.Expectations.Metadata;
 
 namespace Mono.Linker.Tests.Cases.UnreachableBlock
@@ -14,6 +15,9 @@ public static void Main ()
 		{
 			Test_1 (null);
 			Test_2 (9);
+
+			// Prevents optimizing away 'is Type' check.
+			PreserveSystemType ();
 		}
 
 		[Kept]
@@ -100,5 +104,11 @@ static bool IsDynamicCodeSupported {
 		static void Reached_1 ()
 		{
 		}
+
+		[Kept]
+		static void PreserveSystemType ()
+		{
+			typeof (Type).RequiresNonPublicConstructors ();
+		}
 	}
 }