If I update my OS, will my old printer driver still work? Canon driver for my PIXMA MG7550 is not available

Your printer, Canon MG7550, is an Airprint printer and should work with no additional drivers. However, you may have to reset the printing system and re add the printer

Reset the printing system on your Mac to solve a problem - Apple Palaikymas (LT)

I just checked, and confirmed your findings. Canon has not updated this driver since macOS Catalina. However, if your Mac is running Monterey now and it works, then it should be possible that it will continue to work with the update.

You basically have a number of choices, going forward:

• Double-check with Canon support to see if they will be providing an updated driver.
• If not, then:
  • Install the update to your MBP's internal drive, OR
  • Install the update to bootable external drive. You would then boot up your notebook from this drive to "test" this printer for full functionality.
• Don't update macOS. (Not recommended.)
• Replace the printer with a newer model.