I'm a maintainer (one of many) of an open source project, and this topic has been on my mind a lot lately as I review PRs. I am more suspicious of PRs from new contributors by default now. Of course I keep these suspicions to myself, but besides simply reviewing code for all the regular things, I now ask myself "what sort of sneaky thing could they be doing that appears benign on the surface?"
Wasn't a key thing of the xz attack vector that people where encouraged to download the custom source release instead of the autogenerated Github one? I don't know if that is a pattern but it seems like best practices in the (source) supply-chain could prevent a large class of these attacks.
same with npm. i publish releases of my OSS libs to npm, but there's no guarantee that what is uploaded is what you see on github. that's a lot of trust you have to put into my opsec, etc. not good.