Maybe this will help people who are torn between calling it minhag or Maase imahos. A friend of mine once asked a big poseik (it’s been a while & I don’t remember whom she asked) about buying things for a baby before it’s born (I think this falls into a similar category as the other items that have been mentioned). The response- There is no known mekor for not buying however you must ask rishus from the Bubbies as if it will upset them it should not be done. (He did say that if she could do it in a way that there was no way the Bubby would find out and it really made a difference to her, then she could do it)
Perhaps we can say something similar here: There is no known mekor, but there are definitely families in which this is the respected practice. If you are a member of such a family, it might be worth your while to try being careful.