We all know from the news and based on original how nations might drop out of the project if you perform too badly.
What always annoyed me was that there was no way to get them back aka if you finally manage to cover their territory properly that there'd be no way to show that nation that X-Com was on the upswing and would properly defend them.
Imo in a free game the mechanics should optimally always work both ways: You can muck things up but you can work to recover the lost territory. So losing the USA to a mucked up terror/infiltration missions you might be hanging in the ropes but some 600 days later you managed to rebalance the lost income and increase the capability of your forces enough to convince the US to rejoin.
From abscence of any info I assume that nations can only drop out and there aren't any quest/mission chains that might reincrease the relations enough again for them to rethink it? I would love if the later were the case.