Yes, 21.0 supports 40 MHz 5 ms frame at all duty cycles. It was supported in previous released but capped at max of 67% downlink percentage.
Generally 5 ms frame gives increased sector throughput with reduced overhead. However at 30 and 40 MHz due to internal Canopy MAC limitations the throughput to a single Data Channel can actually go down with 5 ms frame.
In general at this time, for 30 and 40 MHz I’d recommend 2.5 ms frame unless you are GPS synchronizing with another 5 ms system. 21.0 made some improvements in 5 ms frame, and we have future improvements planned to further increase it.
For 20 MHz and below, if you are consistently hitting your max Frame Utilization I’d recommend 5 ms frame to give a bump in throughput at a cost of latency.