[API] Deprecate SyncMsg (#286)

SyncMsg.ksyncStyle is the only SyncMsg ever sent.

+semver:major

CHANGELOG.md

@@ -35,6 +35,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Deprecated
 
+- [SIL.LCModel] `SyncMsg` seems to be a relic of TE, which was discontinued years ago
 - [SIL.LCModel.Core] `new CaseFunctions(string)` in favor of the new `new CaseFunctions(CoreWritingSystemDefinition)`
 
 ## [10.1.0] - 2021-10-01

src/SIL.LCModel/Enumerations.cs

@@ -427,8 +427,12 @@ public enum SpellingStatusStates
 	/// <summary>
 	/// Enum listing types of change being made for synchronization purposes. These messages are
 	/// used to indicate the type of change that was made.
+	/// REVIEW (Hasso) 2023.08: it seems ksyncStyle is the only SyncMsg ever sent anywhere (LCM and FieldWorks, at least).
+	/// I also searched github.com/sillsdev and saw matches in WorldPad not listed here that lead me to believe this is a relic of TE.
+	/// REVIEW: Can this enum be removed?
 	/// </summary>
 	/// ----------------------------------------------------------------------------------------
+	[Obsolete("Only ksyncStyle is published as far as I can tell")]
 	public enum SyncMsg
 	{
 		/// <summary>