Is this input:

+Y (mu = 0.1)

a shortcut to represent a sliding shoe support with friction, where the actual restraints present are:

+Y
X (mu = 0.1)
Z (mu = 0.1)

I've come across the first version in a colleague's work, but can't imagine it's equivalent to the second. (If it was, then there'd be no way to input the vertical restraint with friction that the first input actually suggests.)

Is this a shortcut that I don't know, or a mistake?

CHW,

The second is not equivalent to the first.

In the first case, you have a simple vertical support where the shoe will be allowed to slide in the horizontal plane with resistance of friction. I wouldn't say it's a shortcut, but rather it's the right way to model a sliding support. I think this is the support your colleague was trying to model.

In the second case, it appears that you would be modeling some sort of riser support which would allow the pipe to move upward (against friction supplied by the by the X and Z components), but not downward while being restricted from moving side to side.

In other words, they are both valid supports. It just depends on what kind of support you are trying to model.