Up to what frequency can transistor couplers transmit signals?

The frequency bandwidth of transmittable digital signals depends on how LED current IF and load resistance are selected. It also depends on whether the current transfer ratio (CTR) is high or low. For transistor couplers, since the actual value for Propagation Delay Time is in the area of several tens of μs, the bandwidth is in the range of several kilobits to a dozen or so kilobits per second.
For details, refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of Transistor Couplers", section 2-4-3.Setting of Pull-up Resistance RC(max) and 2-4-4. Setting of Pull-up Resistance RC.