Update the migration documentation.

docs/reference/migration/migrate_8_0/search.asciidoc

@@ -36,6 +36,14 @@ The `nested_filter` and `nested_path` options, deprecated in 6.x, have been remo
 `_search` and `_get` requests. Adaptive replica selection (activated by default in this version) will route requests
 more efficiently using the service time of prior inter-node communications.
 
+[float]
+==== Removal of sparse vector fields
+The `sparse_vector` field type was deprecated in 7.6 and is now removed in
+8.0. We have not seen much interest in this experimental field type, and don't
+see a clear use case as it's currently designed. If you have feedback or
+suggestions around sparse vector functionality, please let us know through
+GitHub or the 'discuss' forums.
+
 [float]
 ==== Update to vector function signatures
 The vector functions of the form `function(query, doc['field'])` were