Hmmm, what about
<defaultSearchField> and <solrQueryParser defaultOperator="..."/>? They've been deprecated since 3.6 so I'd guess the same argument applies. So I guess I'll remove them from most of the schema files but leave them in a couple.
Hmm, not sure about this, let me look a bit more. Unlike removing the <fields> and <types> tags, if the code still looks at these it'd change behavior to no good purpose.
Nope, not going here. The code still looks at these and uses them, albeit with a warning in managed_schema that it's overridden by 'df' in solrconfig.xml. So taking these out would be A Bad Thing.