Adding to this: while certs are indeed well-supported by OpenSSH, it's not always the SSH daemon used on alternate or embedded platforms.
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.
You can just replace dropbear with openssh on OpenWRT. That was one of the first things I did, since DropBear also doesn't support hardware backed (sk) keys. Just move it to 2222 and disable the service.
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.
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.