Merge pull request #70435 from CyrusNajmabadi/updateBCs

Update breaking change docs

docs/Breaking API Changes.md

@@ -96,3 +96,9 @@ Now the reason will be reported more accurately as `IncrementalStepRunReason.New
 The value of `Assembly.Location` previously held the location on disk where an analyzer or source generator was loaded from. This could be either the original location or the shadow copy location. In 4.8 this will be `""` in certain cases when running on non Windows platforms. This is due the compiler server loading assemblies using `AssemblyLoadContext.LoadFromStream` instead of loading from disk. 
 
 This could already happen in other load scenarios but this change moves it into mainline build scenarios. 
+
+### Deprecation warning for SyntaxNode serialization
+
+The ability to serialize/deserialize a SyntaxNode to/from a Stream has been deprecated. The code for this still exists in Roslyn, but attempting to call the APIs to perform these functions will result in 'Obsolete' warnings being reported. A future version of Roslyn will remove this functionality entirely. This functionality could only work for a host that wrote out the nodes to a stream, and later read it back in within the same process instance. It could not be used to communicate across processes, or for persisting nodes to disk to be read in at a later time by a new host sessions. This functionality originally existed for the days when Roslyn was hosted in 32bit processes with limited address space. That is no longer a mainline supported scenario. Clients can get similar functionality by persisting the text of the node, and parsing it back out when needed.
+
+PR: https://github.com/dotnet/roslyn/pull/70365