I noticed that during the first time one brings a game online (example: to redeem events), 8 bytes get written to 0x1010 of the save (initially all zero).
Similar to Gen 6&7, these values apparently are tracked by the Mystery Gift server, stopping you from receiving one-time events again, even if you use a save state prior to receiving them. (Game Sync was used then for the tracking as I described.)
I can also confirm when going online with modified save that have said bytes all zero'd out, the server assigns a new and different 8 byte value to it.
As PKHeX has the Game Sync textbox, but doesn't appear to use it, I'm requesting that PKHeX makes use of the Game Sync textbox to display this value, as this value in all likelihood is the Game Sync value.
(I would imagine this value is used in similar fashions as Gen 6 or 7, except for PGL.)