NamedList.toString() does not put a space after the joining commas, whereas AbstractMap does. I think it ought to be consistent. This can matter if you write tests that toString() a piece of a SolrResponse that varies in its use of EmbeddedSolrServer versus other SolrClients that serialize the response. Some custom SearchComponents or whatever might prefer to use a Map and that should ultimately be consistent. I know the assumption is a little brittle but still.
I think 9.0 without a back-port is safe.