Yes, it does have a builder. What is missing are functions like 'encodeUtf8Builder :: Text -> Builder' which can be used to assemble output from smallish Text values in a zero-copy fashion with a guarantee on a large average chunk size (required for few system calls).
Moreover, the new builder infrastructure also provides support for implementing fast custom encodings (mostly used for escaping + UTF-8 and the like). See this thread for some benchmarks: https://github.com/bos/aeson/issues/56#issuecomment-3834829
In the future, I imagine that functions creating sequences of bytes always result in builders instead of bytestrings, as this gives good compositionality + good performance. For this to happen, builders must be provided together with the bytestring library.