It looks like the way Esri prevents this is very simple, you can add a Geoserver WFS-T service to an Esri "map" but you cannot edit it.
In Geoserver, enabling editing means giving "complete" access to the service and setting appropriate permissions. (In my test, I enabled anonymous writes.) There is no separate support for WFS-T, just "WFS"
If you can't put a writable layer into an esri map, then you can't create apps either.
It looks like you can't edit WFS-T in ArcGIS Pro either.
You can use OpenLayers (I've never tried Leaflet) and QGIS to consume most Esri services but not the other way around.
I could see workflows where you could use open source to put data into a Geoserver WFS-T service and then view it in a webmap, for example automated collection via a GPS tracker.