I am not a doctor so I don't know, but who would need puberty blockers after age 18? I thought they were used to keep immobile, mentally challenged and nonverbal children small so parents could take care of them more easily as they age. A practice I do not condone.
I've never heard it that use of puberty blockers tbh. That is ethically dubious.
They are also used in children with precocious puberty - that is onset of puberty at an unusually early age. This is not being banned by the new guidance; it is specifically around the use in gender reassignment following the scandals around the Tavistock clinic.