It's such an underrated feature. It baffles me how people immediately turn to overly complicated solutions solving a problem they don't really have to solve, just because everyone assumes the only way is the default commonly known way. Like OP, people immediately jump to the conclusion you need extra software to manage the keys, rather than using another authentication method natively supported, and keep filling their known_hosts file with junk, making the whole validation process useless because everyone just accepts whatever key the host presents.

It's amazing how simple it is. Developer needs temporary access to debug a web server? Sure, here's your 2h valid cert to log in as the web user on the server, don't even need to actually log into the server to put their key in and then remove it. I mint a cert and it's ready to go on whichever users and servers I specified in the cert. Can't even gain persistence because regular authorized_keys is disabled and we have limited session durations.

I regularly leave people baffled at work because I come up with a clever and built-in super simple solution to something they expected to just slap more scripts and software to work around the only way they know to use the software. Read your manpages in full folks, it'll save you so much work. Know what your software is capable of.

Is smallstep free to self host? Looking at their pricing page it's kind of unclear, and their saas is pretty pricey.