It means he's staying.
There is absolutely no chance Messi is sitting in the stands for his last game as a Barcelona player.
It kind of works both ways really. The arguement it means he stays is pretty obvious. But put it in the context he may be leaving.
No extension has been announced, which I can only assume can only mean one hasnt been agreed. Can't see any reason whatsoever not to rush to announce it the minute it is agreed. Particularly by Barcelona, but Messi too whilo gets to declare his cule love to the fans and club who have been sitting on the worry of his next move for a year.
If Laporta and Messi's mutual admiration is as strong as it is, Messi has been held to the final year of his contract. He has honoured it. If he wants to go to Argentina early, for the club to hold him to the very last dead rubber game seems petty and pointless, may as well say thanks now and let him off early. He isn't getting a proper goodbye with the fans anyway, in the current climate, so what difference does one more away game make. Either way his emotional farewell will be via some form of statement or interview, really no reason to go through the motions one last time. He has kinda been saying bye to the fans the whole year anyway, given he has not committed his future to Barcelona this late on.
He might also not want to risk injury for the copa america and potentially next move.
Either way I see this as a goodwill gesture by the club to a player, on the one hand both parties resigned to his leaving, pn the other both knowing he will stay, pending ironing it out and announcing it.
To me it feels more like the first of those. Personally .