The keywords "adds", "updates", and "deletes" require an array of dictionaries.
The exact requirements for each keyword can be found here:
Attribute rule dictionary keywords—ArcGIS Pro | Documentation
"deletes" just requires the globalID/objectID identifying the records that should be deleted:
return {
"result": result,
"edit": [
{
"className": "OtherTable",
"deletes": [
{"globalID": GlobalID1},
{"globalID": GlobalID2},
{"globalID": GlobalID3},
]
}
]
}
You already know how to load and filter the related features, so you just have to put it all together:
// load related features
var related_features = FeatureSetByRelationshipName($feature, "RelationshipName")
// filter related features
var filtered_related_features = Filter(related_features, "Attribute = Value")
// create and fill the deletes array
var deletes = []
for(var f in filtered_related_features) {
Push(deletes, {"globalID": f.GlobalID})
}
// return
return {
"edit": [
{
"className": "RelatedFeatureClass",
"deletes": deletes
}
]
}
Have a great day!
Johannes