diff --git a/server/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java b/server/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java
index aeab22a6f5f35..c5d5dbec1ef15 100644
--- a/server/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java
+++ b/server/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java
@@ -101,7 +101,20 @@ public static class Builder extends MetadataFieldMapper.Builder {
                 (previous, current, conflicts) -> (previous.value() == current.value()) || (previous.value() && current.value() == false)
             );
 
-        private final Parameter<Mode> mode;
+        /*
+         * The default mode for TimeSeries is left empty on purpose, so that mapping printings include the synthetic
+         * source mode.
+         */
+        private final Parameter<Mode> mode = new Parameter<>(
+            "mode",
+            true,
+            () -> null,
+            (n, c, o) -> Mode.valueOf(o.toString().toUpperCase(Locale.ROOT)),
+            m -> toType(m).enabled.explicit() ? null : toType(m).mode,
+            (b, n, v) -> b.field(n, v.toString().toLowerCase(Locale.ROOT)),
+            v -> v.toString().toLowerCase(Locale.ROOT)
+        ).setMergeValidator((previous, current, conflicts) -> (previous == current) || current != Mode.STORED)
+            .setSerializerCheck((includeDefaults, isConfigured, value) -> value != null); // don't emit if `enabled` is configured
         private final Parameter<List<String>> includes = Parameter.stringArrayParam(
             "includes",
             false,
@@ -115,22 +128,9 @@ public static class Builder extends MetadataFieldMapper.Builder {
 
         private final IndexMode indexMode;
 
-        public Builder(IndexMode indexMode, IndexVersion indexVersion) {
+        public Builder(IndexMode indexMode) {
             super(Defaults.NAME);
             this.indexMode = indexMode;
-            this.mode = new Parameter<>(
-                "mode",
-                true,
-                // The default mode for TimeSeries is left empty on purpose, so that mapping printings include the synthetic source mode.
-                () -> getIndexMode() == IndexMode.TIME_SERIES && indexVersion.between(IndexVersion.V_8_7_0, IndexVersion.V_8_10_0)
-                    ? Mode.SYNTHETIC
-                    : null,
-                (n, c, o) -> Mode.valueOf(o.toString().toUpperCase(Locale.ROOT)),
-                m -> toType(m).enabled.explicit() ? null : toType(m).mode,
-                (b, n, v) -> b.field(n, v.toString().toLowerCase(Locale.ROOT)),
-                v -> v.toString().toLowerCase(Locale.ROOT)
-            ).setMergeValidator((previous, current, conflicts) -> (previous == current) || current != Mode.STORED)
-                .setSerializerCheck((includeDefaults, isConfigured, value) -> value != null); // don't emit if `enabled` is configured
         }
 
         public Builder setSynthetic() {
@@ -188,7 +188,7 @@ private IndexMode getIndexMode() {
         c -> c.getIndexSettings().getMode() == IndexMode.TIME_SERIES
             ? c.getIndexSettings().getIndexVersionCreated().onOrAfter(IndexVersion.V_8_7_0) ? TSDB_DEFAULT : TSDB_LEGACY_DEFAULT
             : DEFAULT,
-        c -> new Builder(c.getIndexSettings().getMode(), c.getIndexSettings().getIndexVersionCreated())
+        c -> new Builder(c.getIndexSettings().getMode())
     );
 
     static final class SourceFieldType extends MappedFieldType {
@@ -313,7 +313,7 @@ protected String contentType() {
 
     @Override
     public FieldMapper.Builder getMergeBuilder() {
-        return new Builder(indexMode, IndexVersion.current()).init(this);
+        return new Builder(indexMode).init(this);
     }
 
     /**
diff --git a/server/src/test/java/org/elasticsearch/index/mapper/SourceFieldMapperTests.java b/server/src/test/java/org/elasticsearch/index/mapper/SourceFieldMapperTests.java
index 433ebc467483d..f683cb60c87c3 100644
--- a/server/src/test/java/org/elasticsearch/index/mapper/SourceFieldMapperTests.java
+++ b/server/src/test/java/org/elasticsearch/index/mapper/SourceFieldMapperTests.java
@@ -12,8 +12,6 @@
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.xcontent.XContentHelper;
-import org.elasticsearch.index.IndexMode;
-import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentFactory;
 import org.elasticsearch.xcontent.XContentParser;
@@ -240,10 +238,4 @@ public void testSyntheticSourceInTimeSeries() throws IOException {
         assertTrue(mapper.sourceMapper().isSynthetic());
         assertEquals("{\"_source\":{\"mode\":\"synthetic\"}}", mapper.sourceMapper().toString());
     }
-
-    public void testSyntheticSourceInTimeSeriesBwc() throws IOException {
-        SourceFieldMapper sourceMapper = new SourceFieldMapper.Builder(IndexMode.TIME_SERIES, IndexVersion.V_8_8_0).build();
-        assertTrue(sourceMapper.isSynthetic());
-        assertEquals("{\"_source\":{\"mode\":\"synthetic\"}}", sourceMapper.toString());
-    }
 }
diff --git a/server/src/test/java/org/elasticsearch/index/query/SearchExecutionContextTests.java b/server/src/test/java/org/elasticsearch/index/query/SearchExecutionContextTests.java
index 9df1dc24c2793..6d671a258c26a 100644
--- a/server/src/test/java/org/elasticsearch/index/query/SearchExecutionContextTests.java
+++ b/server/src/test/java/org/elasticsearch/index/query/SearchExecutionContextTests.java
@@ -381,7 +381,7 @@ public void testSearchRequestRuntimeFieldsAndMultifieldDetection() {
 
     public void testSyntheticSourceSearchLookup() throws IOException {
         // Build a mapping using synthetic source
-        SourceFieldMapper sourceMapper = new SourceFieldMapper.Builder(null, IndexVersion.current()).setSynthetic().build();
+        SourceFieldMapper sourceMapper = new SourceFieldMapper.Builder(null).setSynthetic().build();
         RootObjectMapper root = new RootObjectMapper.Builder("_doc", Explicit.IMPLICIT_TRUE).add(
             new KeywordFieldMapper.Builder("cat", IndexVersion.current()).ignoreAbove(100)
         ).build(MapperBuilderContext.root(true, false));