I was able to get a KAR built for all of "addons" (by stripping out all unrelated bindings), then removed the built in addons feature repo in my test instance. I was able to confirm that installing the Home Assistant binding automatically installs the MQTT binding just as I expected it to. For uninstallation, it works a little differently than I thought. I can easily uninstall the Home Assistant, and if I had not explicitly installed MQTT binding, it gets removed too. If I had explicitly installed the MQTT binding, it gets left. The change from my expected behavior is that if you have Home Assistant installed, then try to uninstall MQTT... nothing happens. Under the hood, it's just de-selecting MQTT for explicit installation, but because it's an implicit dependency of the Home Assistant feature, it gets left installed. I don't know if this one bit of weirdness is worth updating the addons API and the UI to be able to show "implicitly installed; cannot be removed" or something in this state. I can clearly see the state in the Karaf console under feature:list:

Name                                                  │ Version          │ Required │ State       │ Repository                                                      │ Description                                                                                                            ──────────────────────────────────────────────────────┼──────────────────┼──────────┼─────────────┼─────────────────────────────────────────────────────────────────┼───────────────────────────────────────────────────    
openhab-binding-homeassistant                         │ 5.1.0.SNAPSHOT   │ x        │ Started     │ org.openhab.addons.features.karaf.openhab-addons-5.1.0-SNAPSHOT │ Home Assistant
openhab-binding-mqtt                                  │ 5.1.0.SNAPSHOT   │          │ Started     │ org.openhab.addons.features.karaf.openhab-addons-5.1.0-SNAPSHOT │ MQTT Binding

Once the core PR gets merged, I'll install the bundles to my production installation to get more testing. I also have generic MQTT, Homie, and espmilighthub things, so I'll get good testing exposure. (I'm awaiting the core PR, because having on-off core bundles locally is a huge pain, because anytime a feature gets installed or removed it reverts it, and for many core bundles that can cause most/all of your instance to be broken until you do a restart).

Rebased and merge conflicts resolved. 🤞 it passes CI now that the XSD caching is merged, too.

I guess there are certain situations that it can be considered a breaking change. Should we add a upgrade notification to the distro repo? wdyt?

Yes, I agree. If only because it won't auto-select the new feature.