I’m thinking of switching to a bank that uses its mobile app for 2fa. I use a dumbphone flipphone as my daily driver that only has talk and text and is not able to download apps. I do have an old smartphone that is going to stop receiving updates soon. I was wondering if it would be safe to use the mobile banking app for 2fa on the old phone after it stops receiving updates. I would plan on factory resetting the phone and not downloading or using any other app on it (including the browser). It would also have no Sim card in it. I would leave it off and only turn it on and use WiFi for using the banking app for 2fa purposes.
It sounds like you’re planning for your key to stop working any day now.
They’ll probably issue a warning, like “our next update stops support for OS version [X]”, and then you’ll have to get a new phone anyway - but that day might be far away.
I’m certain it’s safe as far as being hacked goes, but I’m not certain that is a good idea for your second factor to be a phone that you don’t use very often, and which is likely to be out of date any day now.