No, I never said you should or shouldn't use sd_notify. I said that Debian and Fedora added it downstream, that it was not a decision of OpenSSH.

Generally, I think using libsystemd is probably a good idea, if you are a C program that wishes to have support for sd_notify. Even better, even before they moved things to dlopen, the dependency chain was very clean; nobody would've batted an eye at the idea that something, even something as critical as the SSH daemon, would have a very popular and well-regarded compression library which is already trusted in many package managers loaded into the process space. There is absolutely nothing unreasonable about considering that to be trustworthy. Your sshd probably has zlib linked in and it is fine.

Seriously. We're not talking about the difference between zero dependency and one dependency, but rather something more like 34 dependencies vs 38 dependencies. Here's my sshd, noting that libtree excludes some glibc stuff by default:

$ libtree `which sshd`
/run/current-system/sw/bin/sshd
├── libgssapi_krb5.so.2 [runpath]
│ ├── libkrb5.so.3 [runpath]
│ │ ├── libk5crypto.so.3 [runpath]
│ │ │ ├── libkrb5support.so.0 [runpath]
│ │ │ │ ├── libkeyutils.so.1 [runpath]
│ │ │ │ └── libresolv.so.2 [runpath]
│ │ │ ├── libkeyutils.so.1 [runpath]
│ │ │ └── libresolv.so.2 [runpath]
│ │ ├── libcom_err.so.3 [runpath]
│ │ │ ├── libkrb5support.so.0 [runpath]
│ │ │ ├── libkeyutils.so.1 [runpath]
│ │ │ └── libresolv.so.2 [runpath]
│ │ ├── libkrb5support.so.0 [runpath]
│ │ ├── libkeyutils.so.1 [runpath]
│ │ └── libresolv.so.2 [runpath]
│ ├── libk5crypto.so.3 [runpath]
│ ├── libcom_err.so.3 [runpath]
│ ├── libkrb5support.so.0 [runpath]
│ ├── libkeyutils.so.1 [runpath]
│ └── libresolv.so.2 [runpath]
├── libkrb5.so.3 [runpath]
├── libcom_err.so.3 [runpath]
├── libk5crypto.so.3 [runpath]
├── libz.so.1 [runpath]
├── libcrypto.so.3 [runpath]
│ └── libpthread.so.0 [runpath]
├── libldns.so.3 [runpath]
│ ├── libssl.so.3 [runpath]
│ │ ├── libcrypto.so.3 [runpath]
│ │ └── libpthread.so.0 [runpath]
│ └── libcrypto.so.3 [runpath]
└── libpam.so.0 [runpath]
└── libaudit.so.1 [runpath]

But there is one thing that is questionable, and that's whether or not downstreams like Debian and Fedora should really be making patches like this that add new dependencies to security-critical programs like OpenSSH. It's one thing if OpenSSH takes on this dependency itself, but downstreams adding it is scarier specifically because it's so unexpected. If you didn't know the way OpenSSH was packaged specifically in Debian and Fedora, you would have absolutely no way of figuring out that liblzma5 could be in its process space and thus it wouldn't be part of the threat model. Certainly the upstream developers would have zero chance of ever noticing this.

Of course, I'm sure Debian and Fedora have their reasons for the specific patch that adds sd_notify support, it probably does improve something somewhere, but this incident absolutely showcases how the consequences of innocuous-looking patches like that can domino into something absolutely devastating in a non-trivial manner. I strongly suspect whatever it improves on would not have been worth it if the backdoor had succeeded in proliferating. Nothing is a panacea, but you definitely gain some advantage by sticking to configurations supported by the upstream where possible.

That said: Debian and Fedora already (largely) dodged this bullet, libsystemd already (even before this news) plugged the hole that made it possible in the first place, and everything is fine for now. Debian and Fedora thus have no real reason to remove this patch other than if their sensibilities regarding it have changed, because the risk that it posed is basically gone now. Now for this specific patch to pose a threat, you'd have to compromise either libsystemd or libcap2. On any distro that uses systemd to begin with, compromising systemd already gets you root, and libcap2 IIRC is maintained by the Linux kernel folks, so of course if anyone compromises that, it's way, way game over already.

Is this some grand lesson about systemd bloat potentially enabling a horrible sshd backdoor? You can read it that way, but it's a pretty silly take, considering all of the factors. But if it makes people who get emotionally attached to hating init systems happy, more power to them I suppose. As for normal risk analysis, there is no particular factor here that completely dominates for what really enabled this. It was a combination of decisions that, on their own, are completely justifiable and reasonable, but when combined, led to near disaster. Not a new story and not one that's going to stop occurring either...