No there isn't a minimum turnout. Sitting out for an election means that whoever does still vote gets to decide the winner. St scale, though, a decrease in voter turnout would signal discontent with the system and the parties in charge. Those parties would likely respond to try to capture more voters and a third party may finally see a window that they could use to raise funds for a legitimate challenge to the two party system.

I have not seen that's happening anywhere. What actually happens is that parties will just focus on whoever votes and ignore the rest.

Sure, maybe if participation dropped suddenly to 20% there would be comments and reactions, but anything over 40% would barely get a few curious articles and no real difference.