Not sure why, they have header pins for the user port and then an adapter if you want to use things that require the original edge connector. I'm guessing (although I'm not sure what those other chips on the adapter do) you could also connect directly to the header for new stuff. https://www.commodore.net/product-page/u64-userport-adapter

Was it the user port or cartridge port where we'd ground the reset line with a paperclip to reset the C64? I can't remember.

They both have one: https://archive.org/details/commodore-64-manual-en-1982/page...

On the user port I managed to short 5V to GND instead :'(

With a username like that, you should know the answer!