What is M73 Q17 S43 G-code command? I know that M73 P19 means "Set completion progress to 19%", and I suspect that M73 R42 means "Set remaining time to 42 minutes", but what is M73 Q17 S43? I can't find description of such syntax.

The command is seen in .gcode files produced by PrusaSlicer.
The M73 Set/Get build percentage G-code is only defined for a selected few printer firmwares.

As you suspected, next to M73 P19 (tell the firmware at what completage percentage the print is) the M73 R42 tells the firmware the left time to completion.

If you look at the description of the M73 G-code, the following parameters may be used:


P: Percent in normal mode
R: Time remaining in normal mode
Q: Percent in silent mode
S: Time remaining in silent mode


So, running the Q and S parameters, is similar to the P and R parameters with the exception for referring to the printer percentage/time when in stealth (quiet) mode.

These modes, normal and stealth, refer to power modes of Prusa printers:


  Normal vs. Stealth mode
  MK3 printers offer two print modes. Normal mode is required for the detection of lost steps (shifted layers), while still being quieter than the silent mode on MK2/S. There is also the Stealth mode, which utilizes Trinamic StealthChop technology, making the printer almost inaudible with the print cooling fan being the noisiest part of the printer. However, Stealth mode does not provide lost step detection.


Stealth mode times can be a bit higher than normal mode estimation times. Estimation is done by the slicer. As Prusa maintains this feature, their times are accurate for their printers, but that does not have to be the case for custom printers. 

To get the current progress, the M73 command is called without parameters:


  Use "M73" by itself to get a report of the current print progress.