This has been stable for a long time, we just missed removing the experimental[] tag in the docs.
@@ -4,9 +4,7 @@
<titleabbrev>Simulate index</titleabbrev>
++++
-experimental[]
-
-Returns the index configuration that would be applied to the specified index from an
+Returns the index configuration that would be applied to the specified index from an
existing <<index-templates, index template>>.
////