Minor Description
Map<String, Object> getAll(Map<String, Object> sink, Collection<String> params)
Is only used by the CollectionsHandler, and has particular rules about how it handles multi-valued data that make it not very generic, and thus I think doesn't belong here. Furthermore the existence of this method is confusing in that it gives the user another choice against it use versus toMap (there are two overloaded variants).
SolrParams toFilteredSolrParams(List<String> names)
Is only called in one place, and something about it bothers me, perhaps just the name or that it ought to be a view maybe.
static Map<String,String> toMap(NamedList params)
Isn't used and I don't like it; it doesn't even involve a SolrParams! Legacy of 2006.
static Map<String,String[]> toMultiMap(NamedList params)
It doesn't even involve a SolrParams! Legacy of 2006 with some updates since. Used in some places. Perhaps should be moved to NamedList as an instance method.