The research does not agree.

"The primary risks of pubertal suppression in gender dysphoric youth treated with GnRH agonists include adverse effects on bone mineralization, compromised fertility, and unknown effects on brain development."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5290172

"GnRHa therapy prevents maturation of primary oocytes and spermatogonia and may preclude gamete maturation, and currently there are no proven methods to preserve fertility in early pubertal transgender adolescents."

https://pubmed.ncbi.nlm.nih.gov/31319416/

I addressed bone density. The 1st paper said it wasn't clear if the effects were permanent. It identified several confounding factors in the relevant study. It went on to recommend monitoring, supplements, and exercise. It also said switching from puberty blockers to cross sex hormones at 14 instead of 16 would help.

The 1st paper said fertility may be compromised "if puberty is suppressed at an early stage and the patient completes phenotypic transition with the use of cross-sex hormones". Not if they decide not to transition. And cross sex hormones can compromise fertility on their own. The 2nd paper said "There is no substantiated evidence that GnRHa treatment for CPP impairs reproductive function or reduces fertility." Because those patients don't take cross sex hormones when they stop the blockers.

Unknown effects on brain development means no known effects. The only study mentioned found no significant detrimental effects.