Searching around, I found this.
https://www.howtogeek.com/369438/how-to-watch-twitch-on-roku/
“When Amazon discontinued the Twitch app from the Roku Channel Store, Roku users who already had the app installed were still able to use it.”
So, if you want to pay to get it back, I guess you should tell Amazon, since they own Twitch. You can also try some of the workarounds on that page, though things like that are less likely to work over time, and that page is over a year old.