Implementing the $search operator in OData will allow a service consumer to specify a search string that can be found in several different fields.

Obviously there would be a performance impact as Business Central service doesn't natively support or queries on different fields, which is likely why the OData OR operator is also not allowed.

I can see this working similarly to DataCaptionFields, where an API Page/Query would have a list of fields specified as being used by the $search operator.