I've tried a lot of times before and none of the damage_source_propertirs predicates have worked. I tried this:
{
"condition": "minecraft:damage_source_properties",
"predicate": {
"direct_entity": {
"equipment": {
"mainhand": {
"nbt": "

"
}
}
}
}
}

Which should detect mobs who were hit by a custom sword. The output is nothing and the log says nothing. If I try this command: "execute as @e[predicate=namespace:custom_sword] run say hi" it doesn't say hi no matter what. I tried multiple damage sources and nbt checks and none of them seemed to worked. I know the syntax is right because normal nbt checks for in predicates look like this and work completely fine. In another note I'd like to say I would really appreciate if you will fix this bug because it opens so many different options up. Thank you for your work.