QGIS used to be able to perform the expression filters of binary spatial operations on the server when the geometry was provided as WKT or in the GML format. This change converts the geometry from being a static variable in the expression to an OGC filter. The WFS URL is then created by combining a bounding box rectangle, layer filter, and the filtering expression.
A feature request computes the expression in advance to find out if it can be performed on the server, then it's stored in QgsBackgroundCachedSharedData where a new download is triggered.
When a filter request is not a pure binary spatial operation, it will not be evaluated on the server.