Thanks for the link to the study nooka. It leaves a lot of questions entirely unanswered, but - given the ethical implications - I don't expect there to be proper studies to answer them in the near future. After all, the wellbeing of mother AND child is supposed to be one of if not the most important factor(s) in this, so it'll be hard to get this past the ethics committees. (For instance, I recently took part in a non-medical research project and even though my kids would have been happy to participate, they weren't allowed as per the ethics committee's decision under 18s were considered too young to give considered consent.)
Btw the case study also states that the transwoman in question was taking not just hormones, but also psychopharmaceuticals on occasion, but there is no mention whether those were avoided during the time that the baby was being nursed (as it is still being fed, it's entirely possible those drugs were taken too, given the reasons for their use and the length of time this has been going on. This would of course be an additional concern).
And the baby has definitely been adopted by the transwoman even though its birthmother was the transwoman's partner. There could be perfectly logical reasons for that (IVF with donor sperm for instance which may mean no parental rights for the non-birth parent in some jurisdictions if they're not married), but given what some trans widows report, there may be another less benign reason.
On a purely scientific level, the question to answer would be if male lactation is capable of the subtle nutritional changes that the female body provides in response to the nursing child. And we know that breastmilk is better than formula milk. Does a male body produce the same quality as a female body? And if it isn't as good, is it as bad as formula or still better? I definitely wouldn't want to see a single baby being denied breastmilk from its birthmother just to satisfy scientific curiosity, so I hope that for this baby this really was the only alternative to formula milk.
Oh and for everyone worried about being prescribed Domperidone here's a link explaining why it's safely used outside of the US, across not only the EU but also in Australia/New Zealand.
ibconline.ca/information-sheets/health-canada-and-domperidone/
In brief: The FDA warning is based on misinterpreting two studies. In one the average age was over 70, in the other it was almost 80. The participants had a number of existing health problems and the authors could not show that Domperidone caused any problems. Safety for babies was not looked into at all (obviously, given that these studies looked into using the drug for gastro-intestinal issues in elderly patients).
The findings actually showed that the females in these studies had a much lower risk which means that given the much younger age of those prescribed the drug for lactation purposes their risk was the same as that of the side effects occuring naturally. It is considered safe for babies, and prescribed in much higher doses in so many countries we would know by now if it wasn't.