backdoor in upstream xz/liblzma leading to ssh server compromise

Honestly, while the way they deployed the exploit helped hide it, I'm not sure that they couldn't have figured out some similar way to hide it in autoconf stuff and commit it.

Remember that the attacker had commit privileges to the repository, was a co-maintainer, and the primary maintainer was apparently away on a month-long vacation. How many parties other than the maintainer are going to go review a lot of complicated autoconf stuff?

I'm not saying that your point's invalid. Making sure that what comes out of the git repository is what goes to upstream is probably a good security practice. But I'm not sure that it really avoids this.

Probably a lot of good lessons that could be learned.

• It sounds like social engineering, including maybe use of sockpuppets, was used to target the maintainer, to get him to cede maintainer status.
• Social engineering was used to pressure package maintainers to commit.
• Apparently automated software testing software did trip on the changes, like some fuzz-tesing software at Google, but the attacker managed to get changes committed to avoid it. This was one point where a light really did get aimed at the changes. That being said, the attacker here was also a maintainer, and I don't think that the fuzzer guys consider themselves responsible for identifying security holes. And while it did highlight the use of ifunc, it sounds like it was legitimately a bug. But, still, it might be possible to have some kind of security examination taking place when fuzzing software trips, especially if the fuzzing software isn't under control of a project's maintainer (as it was not, here).
• The changes were apparently aimed at getting in shortly before Ubuntu freeze; the attacker was apparently recorded asking and ensuring that Ubuntu fed off Debian testing. Maybe there needs to be more-attention paid to things that go in shortly before freeze.
• Part of the attack was hidden in autoconf scripts. Autoconf, especially with generated data going out the door, is hard to audit.
• As you point out, using a chain that ensures that a backdoor that goes into downstream also goes into git would be a good idea.
• Distros should probably be more careful about linking stuff to security-critical binaries like sshd. Apparently this was very much not necessary to achieve what they wanted to do in this case; it was possible to have a very small amount of code that performed the functionality that was actually needed.
• Unless the systemd-notifier changes themselves were done by an attacker, it's a good bet that the Jia Tan group and similar are monitoring software, looking for dependencies like the systemd-notifier introduction. Looking for similar problems that might affect similar remotely-accessible servers might be a good idea.
• It might be a good idea to have servers run their auth component in an isolated module. I'd guess that it'd be possible to have a portion of sshd that accepts incoming connections (and is exposed to the outside, unauthenticated world) as an isolated process. That'd be kind of inetd-like functionality. The portion that performed authentication (and is also running exposed to the outside) as an isolated process, and the code that runs only after authentication succeeds run separately, with only the latter bringing in most libraries.
• I've seen some arguments that systemd itself is large and complicated enough that it lends itself to attacks like this. I think that maybe there's an argument that some sort of distinction should be made between more- or less-security-critical software, and different policies applied. Systemd alone is a pretty important piece of software to be able to compromise. Maybe there are ways to rearchitect things to be somewhat more-resilient and auditable.
• I'm not familiar with the ifunc mechanism, but it sounds like attackers consider it to be a useful route to hide injected code. Maybe have some kind of auditing system to look for that.
• The attacker modified the "in the event of an identified security hole" directions to discourage disclosure to anyone except the project for a 90-day embargo period, and made himself the contact point. That would have provided time to continue to use the exploit. In practice, perhaps software projects should not be the only contact point -- perhaps it should be the norm to both notify software projects and a separate, unrelated-to-a-project security point. That increases the risk of the exploit leaking, but protects against compromise of the project maintainership.