PX4 does accept lower values than 0.6m/s if you force save it via QGC.
But there are some issues that come with a slow land speed: the ground detector is more prone to false positives and negatives. I don’t recall all the details in the land detector procedure, but there are a few things that depend on the land speed. E.g. the no vertical movement condition. If the vehicle is supposed to land slowly, it becomes more difficult to distinguish between still descending and having reached the ground.
Essentially, it works well if you land on level and soft ground. On uneven ground, we experienced the following several times: ground contact is detected when the first part of the drone (say front feet) touch the ground. The motors switch to zero thrust, the back of the drone falls down until it also hits the ground. The vertical velocity registered by the autopilot exceeds the no vertical movement condition (which was really small due to slow landing speed), the motors ramp up again, the drone levels out and landing is initiated once again, resulting in a loop that eventually leads to the feet braking (bouncing drone) unless the pilot is fast enough to take over manually.
Yet still, for us a slow land speed with the potential issues that come with it are so far the better solution than having a hard impact on every landing with 0.6m/s. All the time that the land detector works correctly, the slow landings are really smooth.