This would be a way to automatically assign player_head items using loot table without having to put in individual names of players.
This loot table entry would look something like this:
{
"type": "item",
"name": "minecraft:player_head",
"weight": 1,
"functions": [
{
"function": "set_skullowner",
"entity": <"this":"killer":"killer_player">
}
]
}
Regardless of the entity selector, the function would default to only players and would assign the SkullOwner nbt to the entity selected.
This could potentially be used as a loot table function for a default advancement too. Potentially the "how did we get here?" advancement as it's much more difficult to achieve in 1.13 compared to 1.12 and is a much more reasonable reward compared to just gaining XP.
This has been implemented with the fill_player_head function. :D