One person involved in the project got tired of the ongoing obnoxious behavior of a few kernel developers (as partially documented in that mail), and decided to

One person involved in the project got tired of the ongoing obnoxious behavior of a few kernel developers (as partially documented in that mail), and decided to move on. That's a sad outcome, and reflects badly on the kernel developers in question. It doesn't reflect a maintainability issue with the Rust for Linux project (which has quite a few developers and many more contributors), except insofar as it continues to be an occupational hazard of participating in the Linux kernel community that you end up dealing with some number of obnoxious people.