If Naruto loses the ability he used on Kakashi and Guy after Kaguya's seal, what did Sasuke lose?

Naruto did lose the seal and the power to magically heal others. Naruto basically has very little six paths chakra left and there is not enough to use to transform to it. This is why Naruto does not use it or the truthseeker orbs after the fight with Sasuke. In order to get six paths chakra mode, you need significant power of all the tailed beasts. Naruto only received very small portions of all of the tailed beasts' chakra. After the war, he used most of that chakra. The sage of six paths asked Naruto to keep a little bit of the tailed beasts' chakra so they can meet inside him. So Naruto can use six paths power, but he chooses not to because the sage of the six paths asked him to save a little bit of the tailed beasts' chakra inside him.

The power that Sasuke lost was not any abilities like Naruto, but his chakra reserves is what he lost. During the war, Sasuke could teleport many times and use the Onyx Chidori a couple of times with no expense to his fighting ability and not that much chakra used. In Boruto, Sasuke opened one portal and had his chakra completely exhausted. He did teleport once during the fight with Momoshiki, but he did not use chakra afterwords. In the war, Sasuke could use the Rinnegan abilities nonstop. But in Boruto, Sasuke can use the abilities once or twice and his chakra would be completely exhausted.

So to conclude, Naruto lost his power to heal, and Sasuke kept all of his abilities, but his chakra is drastically lower than when he was fighting in the war.

My assumption would be that Naruto gained that ability in the hand he lost while Sasuke gained abilities in his eyes. Hence the reason Naruto could never fix his own arm even though he could generate Kakashi's eyes

I'm up with an assumption. It's not really clear if Naruto lost his six paths ability, but if so, Sasuke also has some limitations on the usage of his rinnegan. His rinnegan has tomoes, and some state that it's a representation of it's charge. It doesn't seem to lose it's charge until he wields a no tomoe rinnegan in boruto series. So, his rinnegan must have lost it's ability to maintain it's charge.