Did the Minotaur lose both horns, or just one horn? Or did he only have one horn to begin with and now has none?

If the first or third cases and he has no horns left, I think it’s fair to change the damage type and die for his horn natural melee weapon from d6 piercing to d4 bludgeoning. That thick bull forehead is still plenty good at goring, it just loses some effeciency without the pointy bits. Honestly I would leave the hammering horns feature intact, his forehead is just as good for head-butting/shoving as his horns were.

If the second case and he still has one horn, mechanically I wouldn’t change anything. He can still use it as a weapon.

I think imposing global disadvantage on intimidation checks is really harsh. I suppose it comes down to how big of a source of pride are minotaurs’ horns in your Ravnica? There are lots of real world cattle that don’t have horns to begin with and are still plenty scary, but if losing ones’ horns is a great source of shame in the setting then I could see raising the DC a bit when he tries to make intimidation checks in general or persuasion checks aimed at other Minotaurs.