Yes, but this isn’t a broker connection unless you’ve configured it via the UI, so you can ignore it. I think I’ve seen one or two comments suggesting that people clicked on this, removed all the entries in the fields that they see, then save, and things magically started working.
I’m not sure why you’re looking at file formats and GitHub if you already having working Things and Items files from OH2.5 - I’ve literally just copied them over myself. One thing to note is that you can’t have two instances of openHAB with the same Things files running, because your MQTT broker will get confused with the same client ID.
One thing you haven’t mentioned: what is your MQTT Broker? Are you using Mosquitto? Just in case: OH3 doesn’t come with an MQTT broker itself, anymore - you must use a third party one.
This shouldn’t be true - where did you read that?