According to the axis ordering section in the documentation page
https://docs.geoserver.org/latest/en/user/services/wfs/basics.html#wfs-basics-axis, the following assumptions are used:
EPSG:xxxx => longitude/latitude (x/y)
http://www.opengis.net/gml/srs/epsg.xml#xxxx => longitude/latitude (x/y)
urn:x-ogc:def:crs:EPSG:xxxx => latitude/longitude (y/x)
In practice, this doesn't hold for EPSG:xxxx. Try for instance the following request for the default sample data tigeroi:
The result is latitude/longitude. Although this is correct according to the EPSG database, it does conflict with the documentation; I assume the latter should be updated?
Note: to test this, you have to adjust the WFS service to let it use the EPSG notation; by default, the URN notation is used.