I know Tachiyomi (and associated forks) are a popular way to browse and read manga within the community (myself included). This change means that the Tachiyomi app will no longer include access to installation of all but a select few extensions. Specifically, the only extensions allowed going forward are the “bring-your-own” services. You can see the new extension list here.
Based on some of my searching, this change seems to be in response to the threat of legal action by Kakao Entertainment (a Korean webtoon company). Tachiyomi had recently removed a couple extensions from their app (mangadex being one of them), but this change is sweeping.
Going forward, if you have extensions currently installed on your device, they are going to stay there, but will no longer be maintained. So, if something breaks for the service it is communicating with, then the whole thing breaks.
The latest version of Tachiyomi added the ability to add external extension repositories. So, this enables the option of other groups maintaining extensions in the place of the Tachiyomi devs.
Edit: For more context, there are two relevant discussions on the [email protected] community. One about the mangadex extension specifically, and the other about this latest move.
It is maintained by one of the old contributor/maintainer (they still contribute to the Tachiyomi app). I can say that it is the same trustworthiness as before because volunteers (so pretty much the same people) were also maintaining the old extensions repo. It is not easy to access those people directly from the contributors section in the repository, they also cleared the commit history (I think it is to protect from DMCA), so I won’t link a profile directly, but you can get an idea by examining the people on the project’s Discord. You can also review the code of the extensions you will use, the compilation process is quite transparent thanks to GitHub Actions and you will need to trust the extension again after the new extension updates and when it’s time to update, you can review the source code again. I hope I explained it clearly enough. If I said something that was not clear, I can elaborate.