What was happening with Doomfist that got him nerfed?

Actually, people at high SRs suffered Doomfist a lot more.

The hero didn’t have sufficient counter play, so they removed some of his CC, so that people can control their hero at least a little bit.

Basically, if you were half decent at Doomfist, you could climb really hard, and nobody could do anything about it, just because of how his abilities were designed.