Mosh uses AES-OCB (and has since 2011), and we found this bug when we tried to switch over to the OpenSSL implementation (away from our own ocb.cc taken from the original authors) and Launchpad ran it through our CI testsuite as part of the Mosh dev PPA build for i686 Ubuntu. (It wasn't caught by GitHub Actions because it only happens on 32-bit x86.) https://github.com/mobile-shell/mosh/issues/1174 for more.
So I would say (a) OCB is widely used, at least by the ~million Mosh users on various platforms, and (b) this episode somewhat reinforces my (perhaps overweight already) paranoia about depending on other people's code or the blast radius of even well-meaning pull requests. (We really wanted to switch over to the OpenSSL implementation rather than shipping our own, in part because ours was depending on some OpenSSL AES primitives that OpenSSL recently deprecated for external users.)
Maybe one lesson here is that many people believe in the benefits of unit tests for their own code, but we're not as thorough or experienced in writing acceptance tests for our dependencies.
Mosh got lucky this time that we had pretty good tests that exercised the library enough to find this bug, and we run them as part of the package build, but it's not that farfetched to imagine that we might have users on a platform that we don't build a package for (and therefore don't run our testsuite on).
As a non-crypto-nerd: How viable is it to make a “safe” OpenSSL, which just doesn’t support all the cipher modes (?) that the HN crowd would mock me for accidentally using?
The modes of operation aren't the main reason people use OpenSSL; it's the support for all the gnarly (and less gnarly) protocols and wire formats that show up when doing applied cryptography.
Progress is being made on replacing OpenSSL in a lot of contexts (specifically, the RustCrypto[1] folks are doing excellent work and so is cryptography[2]), but there are still plenty of areas where OpenSSL is needed to compose the mostly algebraic cryptography with the right wire format.
Edit: I forgot to mention rustls[3], which uses ring[4] under the hood.
https://guidovranken.com/2022/06/27/notes-on-openssl-remote-...
Note that the bug is only in 3.0.4, which was released June 21, 2022. So if you didn't update to this version, it's unlikely you're vulnerable.