I imagine Twitch will stop operating Korean CDNs. It wouldn't make much sense to geo-block all Korean IP addresses, but with minimum service I expect Korean viewers to drop off pretty quickly. If the networks are as badly configured as the internet articles claim they are, foreign internet uplinks wouldn't be able to sustain decent quality video streams for very long.

VPNs and such will always allow Koreans to watch Twitch content, but by taking money or paying out revenue, Twitch as a company would create certain expectations of service. Now, with no special effort put into Korean service, I expect Twitch to become essentially useless. Perhaps the Korean localisation team will slowly let more and more untranslated English strings slip into the UI, though they may decide to keep the Korean translations for the benefit Korean minorities outside of Korea.

Service providers like anywhere, net neutrality be damned, run Deep Packet Inspection boxes that have no other purpose in life but to negotiate down your connections for whatever you do the ISP finds egregious.

For video, they generally reduce the video quality throughput, eventually reducing bitrate, causing buffering, and generally making your experience miserable unless Twitch, Youtube, etc pay the providers NOT to on behalf of their users. Cellcos particularly do this a lot to reduce quality so people aren't trying to watch 4k streams on a phone just because they can, it'll negotiate them down to what they feel is an acceptable like 480p, usually with some clause hidden in the fine print of your contract about acceptable use.

South Korea simply legitimized all their ISP's doing so, as much as Ajit Pai did repealing net neutrality for American ISP's as their puppet under Trump.