There haven’t been any changes related to attachedTo field since at least 2015, potentially never. It doesn’t seem we ever used parent field for attachments, only for comments where it contains data about a page containing the comment.
May I ask what made you think there was a change and how did you notice it?
Firstly, thanks for confirming this. This is where I have to apologise. I’m sorry to say it seems that I’ve made a massive mistake and wasted your time here. I retract it all. I’ve pointed the finger at Atlassian, when the problem was very much closer to home.
I will delete the above post at some point later so as not to continue the false information.