Keywords => search_string makes sense.
The 'make it even less specific' - which part do you think is too restrictive? I'm fine with less specific wording, just thought what I specified was general enough.
Discussion
For example, if I search for "cat" the relay may also want to send me a note containing the word "cats" and not only notes containing "cat".
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).
We can edit the NIP as time passes and new options are invented.
Or search_term, search or query?