To quote Julia Serrano's excellent writeup on GAC for adolescents:

The “experimental” label is most regularly levied against puberty blockers, probably because the average person isn’t familiar with them. However, they’ve been used to treat precocious puberty since the 1980s (Comite et al., 1981; Mancuso et al., 1989) and to stave off unwanted endogenous puberties in trans youth since the mid-to-late-1990s (Cohen-Kettenis & van Goozen, 1998; van der Loos et al., 2023). For anyone interested in learning more about them, I’d recommend Giordano & Holm’s 2020 accessibly written scientific review “Is puberty delaying treatment ‘experimental treatment’?” as it answers the most commonly asked questions about the method, its efficacy, potential side effects, and so on.

Giordano & Holm’s review also addresses another common claim levied against gender-affirming care, namely, that there aren’t any “high quality studies.” In actuality, there are many high-quality studies: sound methodologies, significant sample sizes, published in well-respected journals, etcetera. When trans-skeptical people argue this, what they really mean is that there aren’t any randomized controlled studies — where neither the doctor nor patient know whether they’ve received the medicine in question or whether they’ve received a placebo. While this certainly is the “gold standard” for medical trials, it is not logistically possible in cases such as this, as both doctors and patients would quickly surmise which group they were assigned to based upon the changes (or lack thereof) in their bodies. The review also delves into ethical issues regarding withholding this treatment that make controlled studies impossible.

The second paragraph delves into the claim that there are no quality studies on the effects of delayed puberty. We actually have a good number of high quality studies, what we don't have are double blind, randomized controlled studies because of the practical and ethical difficulties of doing so. This, of course, gets twisted into labeling puberty blockers as having no evidence or for being "experimental".

What is long term enough for you? A lifetime?