It seems there has been an update to the way QGIS consumes GeoJSON files in version 3.4x
At 2.18.x, a WFS service with GeoJSON output from GeoServer such as:
...can be added simply as a 'vector > file' or 'protocol > geojson'.
However, at v. 3.4.x, the GeoJSON doesn't work by either a file or a protocol (there are two types of GeoJSON in this list, neither work), with the error "Invalid Layer"
Has anyone else noticed this change or have any advice as to how to work with GeoServer WFS services with GeoJSON output?