diff --git a/src/cascadia/TerminalSettingsModel/ApplicationState.idl b/src/cascadia/TerminalSettingsModel/ApplicationState.idl
index 2143cc6dfde..cc76ac87951 100644
--- a/src/cascadia/TerminalSettingsModel/ApplicationState.idl
+++ b/src/cascadia/TerminalSettingsModel/ApplicationState.idl
@@ -7,7 +7,7 @@ namespace Microsoft.Terminal.Settings.Model
 {
     enum InfoBarMessage
     {
-        // We need a sentinel value to map deleted keys to, as we can't actually delete them (GH#16874)
+        // Unknown and deprecated dismissed message names map to this sentinel value.
         IgnoredDeprecatedEntry = 0,
         CloseOnExitInfo,
     };
diff --git a/src/cascadia/TerminalSettingsModel/TerminalSettingsSerializationHelpers.h b/src/cascadia/TerminalSettingsModel/TerminalSettingsSerializationHelpers.h
index 61bcf7243a7..ee8740642ac 100644
--- a/src/cascadia/TerminalSettingsModel/TerminalSettingsSerializationHelpers.h
+++ b/src/cascadia/TerminalSettingsModel/TerminalSettingsSerializationHelpers.h
@@ -588,16 +588,57 @@ JSON_FLAG_MAPPER(::winrt::Microsoft::Terminal::Settings::Model::IntenseStyle)
 
 JSON_ENUM_MAPPER(::winrt::Microsoft::Terminal::Settings::Model::InfoBarMessage)
 {
-    JSON_MAPPINGS(4) = {
-        pair_type{ "deprecated", ValueType::IgnoredDeprecatedEntry },
-        pair_type{ "closeOnExitInfo", ValueType::CloseOnExitInfo },
+    ValueType FromJson(const Json::Value& json)
+    {
+        const auto name{ Detail::GetStringView(json) };
+        for (const auto& pair : mappings)
+        {
+            if (pair.first == name)
+            {
+                return pair.second;
+            }
+        }
 
-        // Keep deprecated keys last, so when they get serialized again they aren't written out
-        pair_type{ "keyboardServiceWarning", ValueType::IgnoredDeprecatedEntry },
-        pair_type{ "setAsDefault", ValueType::IgnoredDeprecatedEntry },
+        // Unknown names may come from another version of Terminal. They must not prevent startup.
+        return ValueType::IgnoredDeprecatedEntry;
+    }
+
+    JSON_MAPPINGS(1) = {
+        pair_type{ "closeOnExitInfo", ValueType::CloseOnExitInfo },
     };
 };
 
+template<>
+struct ::Microsoft::Terminal::Settings::Model::JsonUtils::ConversionTrait<winrt::Windows::Foundation::Collections::IVector<winrt::Microsoft::Terminal::Settings::Model::InfoBarMessage>> :
+    ConversionTrait<std::vector<winrt::Microsoft::Terminal::Settings::Model::InfoBarMessage>>
+{
+    using ValueType = winrt::Microsoft::Terminal::Settings::Model::InfoBarMessage;
+    using Base = ConversionTrait<std::vector<ValueType>>;
+
+    winrt::Windows::Foundation::Collections::IVector<ValueType> FromJson(const Json::Value& json)
+    {
+        return winrt::single_threaded_vector<ValueType>(Base::FromJson(json));
+    }
+
+    Json::Value ToJson(const winrt::Windows::Foundation::Collections::IVector<ValueType>& val)
+    {
+        Json::Value json{ Json::arrayValue };
+        if (val)
+        {
+            ConversionTrait<ValueType> trait;
+            for (const auto& message : val)
+            {
+                // Drop unknown and deprecated dismissals instead of writing the sentinel back to state.json.
+                if (message != ValueType::IgnoredDeprecatedEntry)
+                {
+                    json.append(trait.ToJson(message));
+                }
+            }
+        }
+        return json;
+    }
+};
+
 template<>
 struct ::Microsoft::Terminal::Settings::Model::JsonUtils::ConversionTrait<winrt::Microsoft::Terminal::Settings::Model::ThemeColor>
 {
diff --git a/src/cascadia/UnitTests_SettingsModel/ApplicationStateTests.cpp b/src/cascadia/UnitTests_SettingsModel/ApplicationStateTests.cpp
index f84152ab823..efad720b13d 100644
--- a/src/cascadia/UnitTests_SettingsModel/ApplicationStateTests.cpp
+++ b/src/cascadia/UnitTests_SettingsModel/ApplicationStateTests.cpp
@@ -13,9 +13,6 @@ using namespace winrt::Microsoft::Terminal::Settings::Model;
 
 namespace SettingsModelUnitTests
 {
-    // Covers the workspace-persistence APIs added to ApplicationState:
-    //   SaveWorkspace / RemoveWorkspace / RenameWorkspace / TakeWorkspace /
-    //   AllPersistedWorkspaces.
     // All tests operate on a throw-away ApplicationState instance pointed at
     // a temp directory, so they don't touch the real user state.
     class ApplicationStateTests
@@ -29,6 +26,7 @@ namespace SettingsModelUnitTests
         TEST_METHOD(RenameWorkspaceNoOpForMissingEntry);
         TEST_METHOD(TakeWorkspaceRemovesAndReturns);
         TEST_METHOD(TakeWorkspaceReturnsNullWhenMissing);
+        TEST_METHOD(IgnoreUnknownDismissedMessages);
 
     private:
         static std::filesystem::path _tempRoot()
@@ -56,6 +54,22 @@ namespace SettingsModelUnitTests
         }
     };
 
+    void ApplicationStateTests::IgnoreUnknownDismissedMessages()
+    {
+        Log::Comment(L"GH #16874 - Unknown dismissed message names must not terminate state loading");
+
+        Json::Value json{ Json::objectValue };
+        json["dismissedMessages"].append("unknownMessage");
+        json["dismissedMessages"].append("closeOnExitInfo");
+
+        auto state = _make();
+        state->FromJson(json, implementation::FileSource::Shared);
+
+        const auto serialized = state->ToJson(implementation::FileSource::Shared);
+        VERIFY_ARE_EQUAL(1u, serialized["dismissedMessages"].size());
+        VERIFY_ARE_EQUAL("closeOnExitInfo", serialized["dismissedMessages"][0].asString());
+    }
+
     void ApplicationStateTests::SaveAndLookupWorkspace()
     {
         auto state = _make();