I've been playing around with the 90° shifter concept a little more and I found a reliable and very compact shifter concept with an output perpendicular to the shifter movement. The latter is convenient for gear boxes that use the new rotary catch, for instance this one. Here are some cut-aways that show the inner workings. The concept relies on two ratchets at the top, a 90 degree limiter around the output and two 8t gears in the shifter. While in default (straight up) position, the upper ratchets confine the upper two teeth of the upper 8t gear. When you move the shifter, one ratchet will block the upper 8t gear and force a shift. When you move the shifter back, the ratchet will slip and the 90 degree limiter will prevent the output from rotating back. A nice convenience is that the yellow transversal axle could be rotated 45 degrees without changing the position of the teeth of the attached 8t gear. That way the yellow axle could be directly attached to a 90 degree limiter and used as transversal output. Instructions on Rebrickable.