Should I change it on the NIP? And then the search_string is just a string, not an array.

Also I didn't get where you would put the options, like "option:no_spam". Maybe we should specify that the "key:value" syntax is reserved for options inside the search string, and relays could come up with new stuff later, like "not:keyword" to filter out posts containing these and so on (relays not supporting the "not" keyword should just ignore that, instead of returning notes containing the "not" word, for example).