Would you expect a full refund from BMW if it were Home Assistant that removed the feature?

Your contract is with the entity you paid. So technically, yeah.

It just strikes me as immediately unreasonable from a common sense point-of-view.

I'm also not convinced BMW specifically marketed integration with Home Assistant vs generic integrations with say Alexa or Google Home or whatever Apple calls theirs. And I'll expect the small print to say that third-party integrations are not guaranteed.

In that case, it's not BMW that removed the feature but the downstream independent dependency