What are examples of research topics/areas where ethical concerns have held science back? Are there any? I guess "cloning humans" is such a field?

A lot of medicines work in animal models but then fail in primates or specifically humans. There's some great veterinary drugs out there as a result.

Altering the germ line of a human is illegal in most countries.
Not the USA, where private companies can do it, one owned by Sam Altman.

The current controvery is whether we should allow the development of new organisms that have a opposite chirality in their chemical composition. They could grow out of control with no way to stop. It's called Mirror Life.

https://news.stanford.edu/stories/2024/12/potential-risks-of-mirror-life

Keep in mind, before the Trinity bomb test, a few physicists, including Enrico Fermi, believed the atomic reaction would ignite the earth's atmosphere in a chain reaction. Army said go anyway.

So many medicines are not licensed for pregnant women, not because there are known risks, but just because conducting human trials in that population is difficult ethically.

As a result, pregnant women have access to less treatment options, and may suffer unnecessary pain and adverse health outcomes. Which is also a bad thing, ethically...

There was a pretty high corpus on research done based on nazi science on the effects of temperature on the human body that is forbidden to use as reference. Those reference could help scientific discoveries, but for kinda obvious reasons they can't be used. I said kinda because they were used iirc until the 70's, when people started to question the ethics of using them.

Yeah most replies here focus on human trials and experimentation on living things.

Head transplant/Body transplant.

IIRC it was going to be attempted around ~10 years ago but the head being transplanted had a change of heart (though I suppose he didn't want to have a change of heart would be more accurate phrasing)

Every college chemistry class I've taken (5 now) has had a professor asking why no living thing uses all d-aminos. Kinda sounds like that experiment may answer the question one way or another!

Are you looking for examples specifically about things that someone wanted or considered researching, or anything at all? Because there's an infinite number of things that could technically be researched if people were a freely disposable resource.

I guess there are also things that do not the disposal of humans (directly), that are still ethically questionable. For example research of optimized mass surveillance and adjacent fields.