Is there a slotted yoke piece? I need it for mechanisms such as the Scotch yoke, but I only see workarounds, like this one, where the slot is made with several other pieces.
I don't believe there is a single piece that exists. It's certainly possible to create a mechanism much smaller than the one shown as well as one with fewer pieces.

In theory a single piece would not be possible, the mechanism consists of a minimum of two pieces for it to be possible: a rotating drive and a sliding yoke (as well as several additional pieces to keep everything in place).

Rotational Drive



There are many ways to create a rotation drive, they all consist of an axle (of any length) and can use pulleys/gears, and liftarms cranks.

1 + 2 

For the first two examples I've used a Technic, Pin with Friction Ridges Lengthwise and Towball (6628) but you can just as easily use a standard pin (2780) as it's much more common.

2 

A similar setup to 1 but with two  Technic Cam (6575) pieces.

3 

The third example with two pieces is probably the simplest and most compact, it uses the Technic, Liftarm 1 x 3 with 2 Axle Holes and Pin / Crank
(33299). This setup is limited to movement between five studs (up/down, left/right).

Sliding Yoke



Here are two examples I've made of a sliding yoke. They are identical apart from the beams used. The first one can move five studs and the second can move three studs (possibly the smallest combination). You can of course use any length of beam to create larger yokes.

The beams are connected with a Technic, Pin 3L with Friction Ridges Lengthwise (6558) and a Technic, Pin Connector Round 1L (18654) or you can replace both pieces with a Technic, Pin 3L with Friction Ridges Lengthwise and Center Pin Hole (87082)

There are twelve pieces in both builds. You can reduce this to 10 if you remove the black standard pin, red connector and black axle with a Technix Axle with a stopper such as this piece and a Technic bush (3713).

This build can be reduced to 8 pieces if you make the substitutions shown above.