There's technically no reason OpenSSHd can't also be used in this context. Maybe 2 decades ago there was a legitimate performance/disk space reason which is why Dropbear was preferred for this use case (and the convention remains to do this day), but nowadays the couple megabytes of difference in your initrd between using Dropbear and OpenSSH won't matter.
> There's technically no reason OpenSSHd can't also be used in this context.
For initrd you generally prefer static binaries. Not saying that OpenSSHd doesn't build statically, but having less code and dependencies makes it easier to statically compile.
But yes, technically there is no reason to not use OpenSSHd, but in practice having a smaller and more self contained binary helps considering that you would want the bare minimum during initrd.
