For context, Wuille says he’s just dropping his permissions as he isn’t doing much maintenance work anymore1.
To be clear: I won't stop contributing to code, review, and all the projects I'm involved in. It just so happens I'm not doing much maintenance anymore, so it's time to drop my permissions.
He's not leaving Bitcoin as a developer. "Maintainer" is a more specific role in open source generally, and in Bitcoin specifically - you have special rights on the code repository, to push/merge new code to the 'main'/'master' branch and similar things.
We generally want at least "a few" maintainers, rather than just one, to avoid central points of failure. But we don't really want people to have that maintainer role, if they're not using it, since it's a security sensitive role.
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