Do the unused I/O pins need to be grounded in a device?
Lattice devices have weak internal pull-ups on all I/Os. So the I/O pins which are not used in the design do not need to be grounded or connected anywhere.
Some of Lattice devices have global setting for pull-ups on IOs like On, Off or Bus Hold, and others may have settings for each pin. By default, the I/Os has a pull-up On. For devices that have global setting, and so long the pull-up is either On or Off, you can leave the I/O pins unconnected.
When you use the Bus Hold option on these devices, the bus hold on unused I/Os may cause some additional leakage due to floating state of the IO when device powers up. In that case, it is recommended that you either ground them or connect them to VCC to avoid leakage current. If the device is within the permitted junction temperature range, this leakage current will not affect the functionality of the design. But if this current is causing the device to go out of junction temperature range, then that may affect device performance.