I'm just gonna try explain how puberty blockers work and why they have a lot of very serious risks, building on @piquegender 's comment on the use of puberty blockers in sex hormone dependent cancers.
GnRH (Gonadotropin Releasing Hormone) analogues like leuprorelin mimics natural GnRH but has a much more potent effect. GnRH is produced by the hypothalamus and works on the pituitary gland in the brain, stimulating it to produce FSH (Follicle Stimulating Hormone) and LH (Luteinising Hormone), which stimulate the ovaries to produce estrogen (or in men LH stimulates the testes to produce testosterone).
The GnRH analogues do the same, but with a much more pronounced effect and essentially overstimulate the pituitary gland until it is desensitised and stops producing FSH and LH at all, and there is no stimulation of the gonads to produce sex hormones. This is why the primary use of these drugs are to treat sex hormone dependent cancers, i.e prostate cancer which is testosterone dependent. Before this desensitisation occurs there is a surge in FSH and LH however, which can temporarily worsen symptoms (such as increase in tumour size in prostate cancer leading to other side effects such as urinary retention).
This is the primary and only licensed use for these medicines, they are serious drugs and generally are used when the benefit and risk analysis is a case of life and death, for use in cancers.
There are devastating effects of using these drugs into adolescence, which we know a bit about due to it being prescribed unlicensed for endometriosis. The main side effects are early onset osteoporosis, leading to bone fractures including in the spine with lifelong complications and in some cases irreversible disability.
In osteoporosis bones are constantly breaking down and rebuilding throughout your life. Bone is broken down by osteoclasts (resorption) and new bone is formed by osteoblasts. Osteoclasts can be made to "work faster" by the RANK ligand, which is a protein that binds to osteoclasts and increases bone resorption. Estrogen upregulates production of osteoprotogerin, which binds to the RANK ligand and mitigates that "boost" it gives the osteoclasts, so that bone is not being broken down faster than it can be remade.
With low estrogen this mechanism isn't preventing the bone from being broken down too fast, and so osteoporosis eventually occurs and bone density decreases, risk of fracture increases.
This is also why women who have had a hysterectomy early in life are at risk of developing osteoporosis and the importance of beginning HRT as soon as possible. Post menopausal women are also at risk for the same reason - obviously many women choose not to take HRT after menopause however the risks are greater with earlier age and many decades of bone weakening.
In men some testosterone is converted into bone-preserving estrogen so low T can result in the same.
Bone remodelling is complicated and this isn't the only mechanism which affects it, also involved is the thyroid and parathyroid glands, and hormones calcitonin, PTH and vitamin D.
Basically it's all very complicated and while lupron does have it's uses, it should not be taken lightly and certainly not used in physically healthy individuals, it's not a magical childhood extending potion. There are no such thing as "puberty blockers", they are cancer drugs.
And as an addition - "cis" women aren't getting free boob jobs! If trans people want cosmetic surgery just fucking pay for it like everyone else and u won't have to wait