Twitch Interactive is the content owner, but they provide the API for 3rd party apps, and 3rd party apps like Twitchy exist on Apple TV and aren't getting shut down. Only thing I can think of is the Twitched app must have somehow violated their API rules.
I think you are right. More specifically, I think the TwitchApi.brs brightscript file on the author's github is very revealing with lines such at this:
m.API_RAW = "https://api.twitch.tv/api" ' Twitch Raw (undocumented) endpoint [It's bloody raw - Gordon Ramsay]
m.API_USHER = "https://usher.ttvnw.net" ' Twitch Raw (undocumented) endpoint
m.API_VIZIMA = "https://vizima.twitch.tv/api" ' Twitch keyserver API
The currently approved APIs are Twitch API v5 (Kraken) and the New Twitch API (Helix). The Twitched brightscript use those as well but only indirectly via a middleware server coded in Java. It also doesn't restrict itself to just Kraken and Helix.
My guess is that Roku appeared in Twitch's web server logs making calls to Twitch raw APIs that are intended only to be used by code written by Twitch themselves. It would be around this point in the story when Twitch would reach out to Roku and ask them to stop violating the Twitch Terms of Service by accessing undocumented APIs. Then Roku, not being in control of the brightscright of Twitched and also not being responsible for modifying it could only offer to take down the channel. Considering that Amazon is the parent company of Twitch, I think any scenario were Roku hadn't appeased Twitch would have gone really badly for Roku.
The good news is I don't see yet a reason why a pure Twitch documented Helix API client couldn't be coded in brightscript. The bad news is there is a lot of work to be done to create it and I'm not sure if FrozenIronSoftware was trying to work around limitations in Helix or Roku or if using a raw/undocumented API was just more convenient.
It would be nice if Roku communicated just a little bit better about the specifics of a change instead of pointing to a canned knowledgebase response. But at the end of the day the canned response really is largely correct and talking in more direct terms about specifics is not going to get us what we want.
If you are missing out on the toxic trolls with guns in Fortnite, you could try the NRA TV channel which (in my somewhat joking opinion) also provides toxic trolls with guns. That channel also has the benefit of having generated drama on the Roku forums.
Or you could try:
(1) Miracast'ing Twitch from a PC to the Roku (which I have done)
(2) or paying for PlayOn channel/app (I have never used it myself)