My interpretation of True Polymorph is that, once made permanent, it could not be undone (except perhaps with a Wish spell, god-level magic, another casting of True Polymorph, or some similar ability). Spells like Dispel Magic would not affect a permanent True Polymorph.
However, in the comments of this question, dispelling True Polymorph is mentioned/briefly discussed. So, can any item or spell dispel True Polymorph once the transformation is permanent?
The linked question concerns whether the spell will naturally end, or if altering the polymorphed form (such as cutting the chunk of adamantine) will end the spell. The answers do not discuss the concept of dispelling the polymorph. My question is all about dispelling a permanent True Polymorph, and arguments about the spell ending through damage or other non-dispelling effects should not be considered here.