The Shift constrain feature was replaced in R8 by new functionality in Lazy Mouse 2.0 in 3d edit mode.
The new functionality in 3d mode is:
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Make sure Lazy mouse is active. Make contact with a surface (pen-down for a starting point)
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Hold down shift, and drag out the guide line to an end point. Note the number indicates the angle degree, so a “zero” would be a right angle.
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Release Shift while still holding the pen down, and the line will be drawn.
*Increasing the Lazy radius to 2 or greater enables a 90 and 45 degree snapping guide.
In Projection Master, the functionality is similar but not quite the same because Lazy mouse doesn’t work with the old 2.5d brushes. You must click/make contact with the pen first, THEN hold down shift and move in a 90, or 45 degree direction.
Note that the constrain is not absolute, and it is easy to deviate from the line if your stroke is uneven. 45 degree strokes in particular can be tricky. If all you were using PM for is to make straight line deformation, I would do this in 3d mode now instead.