OP, there are some great links here with evidence.
To add another thought if needed, would the person you need this for consider a sports bra instead?
If no, the next "best" alternative could be sports tape. I discussed this with a medical professional who I trust (who has been on the journey with me supporting my daughter) when my daughter told me that she didn't like sports bras "because people can see the outline of them". He was well aware of the problems with breast binders and that they were harmful. All types. There is no such thing as a safe binder.
We tried the sports tape for a while (even thought I was reluctant, because it is a potential foot on the affirmation pathway) but in the end my daughter concluded that a) she didn't like having to keep replacing it and b) the expense wasn't justified. I had previously put my foot down on "trans tape", her original request as a sports bra alternative, and had explained to her that I wasn't prepared to support anything that was branded and positioned as something that was specifically for flattening breasts in relation to gender identity. It was an incredibly difficult conversation - at her suggestion we booked an appointment with the health professional and he facilitated it - but we worked through it successfully together.
She is now happily using sports bras again. I think it helped that she found out male footballers use them too (to hold heart rate monitors etc), so perhaps it doesn't really matter if people see the ridge of the outline.
Obviously please ignore if it's no help, as I appreciate the ask was for info about harm.
In a nutshell, my approach has been to:
a) recognise that my daughter doesn't like her breasts at the moment and understand how she can feel more comfortable about them
b) draw the line at anything which introduces the idea that her discomfort is a sign that she isn't meant to be a girl. First and foremost, she is an autistic girl who is struggling with her changing body during adolescence, both in regards to the changes that are out of her control and in a sensory aspect. As her mum, I want to support her to feel more comfortable in herself, based on the reality of her body and without introducing any bias.
Flipent I fully agree that mental health should be a factor, hence my approach above. However, I don't see any value in providing a short term fix, which is the start of an affirmation pathway that is most likely to lead to breast amputation. I looked at what was said online in "LGBT communities" about breast binders. The takeaway message was that they hurt and cause all sorts of problems but that's OK because when you're ready for "top surgery" all your problems go away.
There is no evidence to suggest that any of this does actually resolve the underlying mental health issues.... and plenty of growing concern that it does the very opposite.