That is going to hurt a lot of people as some were making real good money on Twitch. I am thinking of popular streamers as Xposed, Ayezee, Foss, ... They also stream on Youtube but the go platform has always been Twitch.
Twitch was also making money through them. More traffic, more donations, more bits bought and so on...
They're cutting their own revenue for nothing because it's not like they're against gambling since they're allowing other forms of gambling like sport bets and people are just going to move somewhere else.
It's not going to stop gamblers and it's not going to stop streamers. Pretty pointless if you ask me.