When transhumanism in the context of Star Trek is brought up, it is seen as a mistake not to include it more thoroughly, or as something perhaps necessary from a storytelling perspective but not true to real life. When someone takes an in-universe approach, they make it out to be a consequence of a psychological scar left by Khan and co. I think that this is not generous enough to the Federation or to the rest of the Trek universe for that matter.

When people ask questions about transhumans in Star Trek, the general phrasing is "why isn't there any?" I would like to pose the opposite question, why should there?

Proponents of genetic engineering are able to produce a laundry list of things genetic engineering can lead to, smarter individuals, with more tuned senses or other advantages over us mere mortals. Shouldn't we want that? Pretending for a moment we had no history of how it goes wrong, isn't it desirable?

I'd suggest that no, it really isn't as great as it is made out to be from a practical perspective. Genetic engineering at its best in Star Trek produces beings of intelligence similar to the upper crust of the Federation's non-augmented population. In Star Trek don't forget, Julian Bashir, our only 100% positive example of human augmentation, was salutatorian of his medical class, and who's to say that in the average situation (not the USS Enterprise's "average situation") a non-modified individual won't to do just as well? Essentially genetic engineering produces good individuals but it still produces people within the normal range of human ability. We do know that genetic engineering can produce individuals far better than ordinary in other ways, they can be far stronger for example (see: Khan). However, why does the Federation care about making its citizens stronger? Unless they start producing a warrior caste, you're just going to end up with a bunch of unreasonably and unnecessarily strong people, I guess you could argue better capable of defending themselves, but that's hardly that important for most Federation citizens.

Up to now, I've been approaching this from a Federation-as-a-whole perspective, that's because this is exactly how it is to a great extent within the Federation. Sure, people want their children to do well, but for the most part there would be social stigma against unnecessarily competitive parents who want to make their children into mini-Bashirs/Khans. Just like we look down on parents today who force their kids to do a whole bunch of tutoring and extracurriculars at the cost of a childhood. This is seen as unacceptable, looking at it from a Maslow's Hierarchy of Needs perspective, the goal should be to be self-actualized for people of the Federation, not to beef up their esteem compared to the masses or their security compared to a perhaps weaker individual. It isn't difficult to see how this, on top of a little preexisting bias, would stop any unnecessary genetic engineering and enshrine it in their laws.

Now, let's get to the ace-in-the-hole for anti-transhumanism policy, the mental deterioration of the genetically engineered. In humans, we know of in canon (AFAIK, haven't seen any pre-TNG era Star Trek, an unfortunate oversight on my part) one individual who was genetic engineering and came out of it unscathed. Every other example, Khan's batch of augments and the ones Bashir collaborated with on DS9, are all dangerous and jeopardize the security of the Federation. Not only does the Federation not want to allow the mental health and security crisis that might follow from legalized genetic engineering, it also does not want to even begin experimenting with that sort of thing. It is unethical, especially for the Federation, playing the odds with the life of a child hoping to find a way to make another Bashir and fewer Khans.

Despite this there is still a case of officially sanctioned genetic manipulation in humans, the children with overactive immune systems in TNG. This shows that it isn't just some irrational fear, it is a rational decision that the Federation is willing to compromise on, even then it shows once again, our attempts to improve humans have unexpected and dangerous consequences. I'm not sure that something like the Darwin Station will ever be allowed again given that once again, genetic research led to disaster.