OpenSSH /
RSAkeys
If you cannot use your ssh-rsa key anymore since you upgraded to Openbsd 7.0 or higher, than here is the solution:
Good Solution
According to https://www.openssh.com/txt/release-8.8
"Incompatibility is more likely when connecting to older SSH implementations that have not been upgraded or have not closely tracked improvements in the SSH protocol."
So Upgrading PuTTY (or any other client that doesn't implement openssh 8.8) to the latest version is the solution.
Bad Solution
But if you really want to not update or if you cannot update, then try this:
Just add to you /etc/ssh/sshd_config the following:
HostKeyAlgorithms +ssh-rsa PubkeyAcceptedAlgorithms +ssh-rsa