The new advancement trigger "minecraft:killed_by_crossbow" does not appear to work correctly when using it for custom advancements. While vanilla advancements that use it (namely, "Who's the Pillager Now", "Two Birds, One Arrow" and "Arbalistic") work properly, if you try to create a custom advancement or modify the vanilla advancements to use the trigger, even if you use the exact same json file but in a datapack, it does not work at all.

For example, if you have criteria with code simply as follows:

"criteria": { 
    "crossbow": { 
        "trigger": "minecraft:killed_by_crossbow" 
    }
}

Then kill any mob by shooting it with a crossbow, notice you do not get the advancement.