How to exclude certain fields from .search()
Starting with Craft 3.1 the solution is easy. In the field definition there is a new checkbox: "Use this field's value as search keywords". Uncheck and you're done!



Please note that there was a bug until Craft 3.1.9.1 which prevented this from working. Thanks to Romain poirier and Brandon Kelly (https://github.com/craftcms/cms/issues/3837).