I've set up a brand new computer with PowerShell 7 and posh-git. When trying to run profile.ps1 on startup the Start-SshAgent command fails, resulting in cmdlet not recognized. Import-Module posh-git works fine, I see the command line prompt changes. In fact, everything else in profile.ps1 from my old computer is working fine.
I tried checking that my environment variables are set up. I'm able to run ssh-agent.exe from the PowerShell prompt, so that doesn't seem to be the issue.
SSH functions have been moved to a separate module: https://github.com/dahlbyk/posh-sshell.
(You might mean Start-SshAgent rather than StartSsh-Agent.)