don't cause any irreversible effects, right?

From the Mayo Clinic:

Puberty blockers can be used to delay the changes of puberty in transgender and gender-diverse youth who have started puberty. The medicines most often used for this purpose are called gonadotropin-releasing hormone (GnRH) analogues. Here's a summary of their possible benefits, side effects and long-term effects.

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Puberty blockers are not recommended for children who have not started puberty.

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Are the changes permanent?

GnRH analogues don't cause permanent physical changes. Instead, they pause puberty. That offers a chance to explore gender identity. It also gives youth and their families time to plan for the psychological, medical, developmental, social and legal issues that may lie ahead.

When a person stops taking GnRH analogues, puberty starts again.