If she's confirmed to be dead, there's some sort of confirmation which can in turn be cited. You learned this from something somewhere, unless Argentinians just magically know when one of their own dies.
Whenever someone actually looks into this exact subject (because this is a surprisingly recurring topic) they end up finding Marcelina Cordoba's obituary and mistaking it for Mariana's due to their similar names, finding the inconsistent reports from Ana Mancini who keeps waffling on if Mariana died in 2014, 2015, or 2016, finding IAFD listing her as dead with no cause and a date of death in conflict with Ana's claims, and finding random people in forums much like this one who claim to know she's dead just because.
I'm pretty sure she retired or died, given her apparent health issues and inactivity, but I wouldn't claim to know which since nothing verifiable has ever circulated online, just unsubstantiated and conflicting claims.