Is printf debugging over JTAG (not SWD) with STM32 possible?
No.  JTAG requires that the clock stops, and then you dump the registers and other states.  In the similar question, that was using USB so you could get information from the running system by sending text via CDC, since it looks like a serial port.

I haven't used the most recent IAR workbench, but due to the architecture of JTAG, you would probably have to get "printf" style information out of the UART.  You could use JTAG to copy information to a spot in memory, and then dump that memory segment when the clock restarts.