It should be remembered some children receive puberty blockers because of early onset puberty. So some girls may be on them e.g. 8 to 12 years of age. This is I think an appropriate use. Puberty blockers don't permanently block puberty, they can be stopped and puberty triggered. Puberty in girls historically could be as late as sixteen, there may be a case for deferring puberty in those with severe gender dysphoria, perhaps allowing time for other therapies to work, or for a clear and informed decision to be made about transitioning.
The impact for precocious puberty is extremely different than for a natural puberty. Puberty is a two-part process that involves both the pituitary and the adrenal glands. GnRH agonists *only* halt the pituitary, they do nothing for the adrenal. The pituitary process during puberty governs bone density accretion, maturation of the sexual organs including breast growth in females, and
increased coarseness of pubic, underarm, and leg hair for both sexes. It also triggers the growth of facial and body hair in males, as well as thickening of the voice box resulting in a lower voice. The adrenal governs lengthening of the long bones and closure of the growth plates after a certain period of time, as well as the initial growth of fine pubic, underarm, and leg hair.
In precocious puberty, the blockade stops the pituitary process until the adrenal kicks in. This prevents bone density accretion in bones that haven't lengthened, and prevents deformation of the legs and arms. Halting puberty in precocious puberty is 'reversible' largely because the adrenal triggers certain stages in the pituitary process, including the "halt" signal for bone density, and a few others that I'm not as conversant on. Once the adrenal has started its process, the child can stop taking the blockers and a normal puberty resumes until the adrenal sends some of those very important "stop now" signals.
In a normal puberty, it's not reversible. The adrenal has already started its process, and GnRH agonists don't affect it at all. If they're taken for a very short period of time (a few months at most), the impact is small and probably not very risky... but it might also depend on exactly where in the process the child is. Bear in mind that one of the more blatant issues is that the adrenal will cause the long bones to grow... but the pituitary isn't on-line to increase bone density. That's a time-bound process - kids can only acquire density in that window. If you stop taking the blockers partway through, the kid will have some increased bone density, but are likely to be permanently osteopenic. If blockers continue until after the adrenal causes the growth plates to close, they will never be able to gain density and can have permanently osteoporosis from before they're even fully adults.
The claim that blockers are temporary or reversible is perhaps the worst myth out there. They really aren't. And that's before we even get into the side effects of those drugs, which aren't something to scoff at.