Republicans stopped being libertarian when they started supporting Trump.

Republicans were a coalition of anticommunists. This resulted in neoconservatives, paleoconservatives, and libertarians coming together. When the Soviet Union fell the coalition started to fall apart leading to fewer libertarians supporting the Republican party since the threat was less. The neoconservatives and paleoconservatives remained in the Republican party.

Republicans were never libertarian in any meaningful sense. There's a reason libertarians have their own party.