I have IKEA Trådfri (which is ZigBee-based). While the hub does connect to the internet for software updates (using ethernet - no WiFi, and no WiFi password to update), there's no "cloud" component. You can't create an "IKEA account", even if you wanted. If IKEA went out of business tomorrow, my switches and bulbs would keep working. I like that approach a lot.
It would be perfectly feasible to leave the IKEA hardware in the house if we were to move.
IKEA are one of the outliers in this space unfortunately.
Last year I bought some Osram Lightify bulbs, as here in Europe they are compatible with the Hue hub (which like Tradfri also works offline). I also got their own hub to update the bulbs, and it is absolutely useless. It says it is connected locally, but still it takes seconds for the bulbs to change state, and usually in a group only some of them change. The same bulbs paired with the Hue hub work perfectly.
It is WiFi only, and I‘ve had it randomly lose the connection and need to be reset. I haven’t tried it offline, but I imagine it’s not going to work.
I only got smart bulbs as I wanted to be able to change the colour temperature and brightness, they are just connected to dumb switches. Tradfri seems superior for this now, as their controllers don’t even need a hub to control lights.
Works great so far. The iOS app was janky at first (it would forget about the hub it was connected to) but seems better now. I don't really use the app though - a recent update added Alexa support, and that works great. I've mounted the remote control on the wall and either use that or Alexa to turn the lights on and off.
I don't want purple or green lights in my living room, so the fact that IKEA bulbs are limited to warmth and brightness is fine for me (I like to just keep them on "warm").
It would be perfectly feasible to leave the IKEA hardware in the house if we were to move.