I don't know that any stock Android swaps to disk. They swap to zram, generally. That said, the thing that seems to make old Android devices obsolete is Google

I don't know that any stock Android swaps to disk. They swap to zram, generally. That said, the thing that seems to make old Android devices obsolete is Google Play Services. It takes up significantly more RAM over time, and it keeps getting updated even when your OS isn't. A degoogled older Android device stays about as useful as it ever was.