You should use the "Arduino Mbed OS Nano Boards" instead of the deprecated one.
The reason for the deprecation is that, as more and more new boards were added to "Arduino Mbed OS Boards", it got huge and installation, updates, and compilations were getting slowed down. So the developers split it up into separate board "families". So using "Arduino Mbed OS Nano Boards" will be beneficial even though the deprecated one will still work.