I think it's completely alright to be curious about future updates, especially after that amount of time. But I mean...you could've just asked, like "Hey, what's the current state of development" or "When do you plan for the next update to release" or something like that.
That would've been far more respectful than being demanding or judgemental about it right away, without knowing the reasons for the delay.
And about your worries for the donators: They can choose to stop donating literally anytime, it's up to them if they wanna believe in an update or not or if they maybe just donate out of appreciation for what's already there. It's not really an unjust thing for the developer to receive donations, even when there's no new updates. In the end, every donator needs to decide that for themselves.