Overwatch was close to or ahead of League in terms of playtime in Korean PC bangs for about a year after launch. A few months back, Blizzard implemented a change that made it more difficult to smurf at PC bangs, presumably to help deal with the aimbot issue, which reflected in an immediate decline in playtime.
Since it caught on, PUBG's been eating Overwatch's lunch in Korea as it has the rest of the world. At the moment, Overwatch is at 13% (which is probably a historical low - although I can't look at historical data without signing up for an account, it wasn't that long ago that it dipped below 20% for the first time). For comparison, League is at 26% and PUBG is at 19%.
Twitch viewership is down a nonnegligible amount, probably on account of PUBG. On a 3 month timeframe (compared to the preceding 3 month window), viewer time declined somewhere around 20% and average viewership for all channels combined has dropped about 50%. Twitch as a whole grew during that period, so that doesn't account for shrinkage.
The good news for Overwatch is that it's not likely to slip that much further on Twitch - the numbers have looked pretty stable on shorter timeframes, and it's pretty reliably in 6th behind the new competitive big 5 with games below it or temporarily surpassing it (Destiny 2, GTA5, WoW, FIFA, Fortnite) unlikely to leapfrog it in the long term. Nonetheless, I think Blizzard shot themselves in the foot on this front.
