All of Canon's EX and EL speedlites are compatible. Except for the new EL-5 speedlite.
Yongnuo YN 600EX-RT II: As of my last update in September 2021, there have been user reports of compatibility issues between this flash and the Canon SL3 due to the difference in pins. This might have changed if Yongnuo released updates or new revisions of the flash. Godox TT685C (for Canon): Godox tends to update its flashes' firmware more frequently than Yongnuo, which means there's a better chance that the TT685C is or will be compatible with the SL3. Still, it's always wise to double-check the latest user reviews or contact Godox directly for the most recent compatibility information.
Godox speedlites are also incompatible. They'll only fire the E-TTL pre-flash. The main flash doesn't fire. More than likely due to the center contact missing. 3rd party speedlites may rely on that pin. But Canon's speedlites may have never needed it from the beginning.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.