Looking for this myself. Just came across this github repository for installing Twitch.tv, a previously discontionued app for Tizen: GitHub - fgl27/smarttv-twitch: Twitch client for Samsung Smart TVs 2015 and newer models, in doubt read the README bellow.
It’s complicated but very well written.
What I have found is that you can compile your own. It will not let you install third party apps directly from TV.
You need to download tizen studio, link your TV to your compiler, and copy and compile code.
So far I have not come across a collection of these so called open source code repos.
The fact that you can add your own apps to your smart tv makes me believe there is still hope. Might make my own collection if I can’t find anything.