For example, OpenWRT used Dropbear [1] instead, which does not support certs. Also, Java programs that implement SSH stuff, like Jenkins, may be doing so using Apache Mina [2] which, though the underlying library supports certs, it is buggy [3] and requires the application to add the UX to also support it.
[1] https://matt.ucc.asn.au/dropbear/dropbear.html
[2] https://mina.apache.org/sshd-project/
[3] I've been dealing for years with NullPointerExceptions causing the connection to crash when presented with certain ed25519 certificates.
I reenabled DB on that alt port when I did the recent major update, just in case, but it wasn't necessary. After the upgrade, OpenSSH was alive and ready.
You can setup your security any way you like, but nothing beats an air gap in terms of security and simplicity.