Maybe Viktor wasn't OP, maybe it was just the build?

It's his Q.

Low CD, further reduced through cheap CDR from Gauntlet. Slow from Gauntlet to permanently lock down enemies and hit the rest of your skills without any problems. High shield uptime while having strong chase potential.

It's basically the same issue that Ornn also has (or had now that they are going to remove his W shield). You can take trades because your damage and shield are the same ability and up all the time. There is a reason they didn't give him a reliable slow on his own kit. Vik loves Rylais because it helps him hit his skills, especially the ult. The high cost and not so great efficiency of the item itself however were keeping that situation somewhat balanced. Gauntlet on the other hand doesn't have the draw backs but the same utility + armor being more valuable than HP make the item many times better.

Viktor was never OP - not even under the radar. Him being picked in worlds as a specific counterpick brought him to the public eye, which made him flavor the month. Fact of the matter is, most of top lane actually counters viktor pretty hard. If youre mobile and can stick to him, you can very easily kill him. Like all battle mages. Fiora, Riven, Ekko, Akali... all good champions against Vikky.

I don't think Viktor is OP, I think he is just good against what's good in top lane, Viktor mid is garbage tier, the only reason Viktor top is good because he can take klepto and farm with-out getting poked and getting his tier 2 upgrade early from the free gold, and abusing champions that can't easily gap close him.

Irelia and Jax are both easy counters to him because Irelia can shred his shields and jax can counterstrike his Q auto damage and since he maxes Q first you essentially gut his damage.

Many mid-range mages suffer in mid lane where they can be out ranged by artillery mages or out bursted by assassins.

Mid range mages always create a niche for themselves in top lane when strong.