Are champs nerfed when they are OP in diamond but balanced everywhere else?

they are nerfed when they are too popular in LCS or overpowered in urf, infact they just disable sona from urf

If it's just in diamond then no, if it's in diamond + obviously yes

riot is balancing around pro play and everything plat and upwards in solo que.

plat is where they compromised and drew the line of people still learning the game and improving themselves mechanically on the champions theyre playing, and people who "mastered" the game to a statisfying enough degree for champion winrates to be an accurate reflection of that champions strength more than a reflection of players making strategical and mechanical mistakes.

it probably also has to do with plat and upwards still containing enough players, and games to collect enough data to make statistical analysis statisfyingly accurate enough.

they only ever balance around lower divisions when a certain champion is severely out of line with far above 55% average winrate in bronze, silver and gold.