There wasn't really any indication on the website that there would be maintenance when I logged in. It also doesn't say how long it was going to last. Was hoping to get a good grind in today with the cards I rented last yet and hope the timers get extended with maintenance.
Splinterlands announced the downtime on their blog and a pop up message did appear on the Splinterlands site warning about the maintenance just before it started. There's no time frame as to how long the maintenance will last.
I doubt there will be an extension on rented cards as those cards are privately rented from other players. The site itself only provides the means for players to rent to each other and takes a 5% fee for providing that service.
The good news, these applied updates should mean smoother game transactions.