I got another update from build 4190 to build 4200 (still version 9.4.0) and my Angry Birds is also still broken. I saw on another thread someone mention a version 9.4.1 starting to roll out but I haven't received it and who knows if it will fix it. I also saw some users recently mentioning Angry Birds as if it was still working 2 weeks ago so not sure if maybe they don't have the update or if the broken app is a local device issue that can't be fixed anyway as I see many warnings that if you uninstall to there is no way to reinstall as the app is gone from the Roku channel store.
If any of you are itching for an Angry Birds fix, it looks like it still works on PC even though the software is over 8 years old now. I purchased the "3 in 1" CD-rom version of the game on ebay still new and unopened for around $5 and it appears to have installed and activated just fine on a windows 10 PC, even though it's no longer officially supported by Rovio. I just played a few levels and it's the same game as on the Roku 3 (Space) and the mouse control is a similar experience as the Roku gaming remote (also no longer made). It also comes with two other games, the regular "Angry Birds" and "Angry Birds Seasons". The windows 10 Microsoft store also has a couple of Angry Birds games that are downloadable and playable for free.
Edit: just noticed not all game levels/worlds are on the PC version of Space and with no updates it's probably not ever going to get them even if you complete the available levels. Oh well, I guess the game is really dead and they really don't want you to play it as they could have easily just set something up to let people just play all levels as freeware after it had been officially discontinued in 2015.
What bugs me is that Roku basically does not acknowledge Angry Birds in any clear way even when they somewhat discontinued it 6 years ago, and it was a "special edition" with even the boot screen featuring the game! It's upsetting when you buy something and it turns into something else but I guess that's how it is with anything software. I will definitely miss the game, it was my favorite feature of the old Roku 3 which I am replacing soon with an Ultra LT.