I first ran into this error after installing Solaris 10 on a Sun Ultra 60 that I had a while back, but I’ve recently ran into it again installing Solaris 10 on Proxmox:
Unable to negotiate with 192.168.1.95 port 22: no matching key exchange method found. Their offer: gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
The solution to configure my Mac to be able to use older key algorithms was a combination from here and answers on this post.
I edited my ~/.ssh/config and added a Host entry for the ip of my Solaris 10 instance, adding:
Host 192.168.1.95
HostKeyAlgorithms=+ssh-dss,ssh-dss
KexAlgorithms +diffie-hellman-group1-sha1
And now I can just ssh in as normal (no need for any additional params as shown in the first post, as the required options are configured in my .ssh/config.