Match events without an 'additional fields' entry

You could try ; $e.additional.fields["LOCALIZED_URL_CATEGORY"] !=/_{1}/ , this will exclude all entries with only one underscore “_” .

Thanks! 
Tried that and I still get events where there are categories as well as events without categories. 

I’m thinking the field isn’t really searchable?

No it is searchable even using the key-value pairs separately, e.g; additional.fields.key = "LOCALIZED_URL_CATEGORY" AND additional.fields.value.string_value != /_{1}/

Could you share a sample of the field values from the query $e.additional.fields["LOCALIZED_URL_CATEGORY"] !=/_{1}/   ?
Is this field multi-valued ? 

Oh, this worked; pulled events where there is no category: 

metadata.product_event_type = "PASSWORD_REUSE"

AND

additional.fields.key != "LOCALIZED_URL_CATEGORY"

Thanks for the help!