On a fresh amd64 stage3:
# emerge -v1 net-analyzer/munin
# emerge --config net-analyzer/munin
Generating public/private rsa key pair.
unknown key type ecdsa
Ideally the munin config should catch the failure and fall back to just RSA keys. See #390937, #402441, and #488814 for more about openssh[-bindist] not supporting ECDSA keys.
Created attachment 366406 [details, diff]
Untested patch for handling missing ECDSA functionality.
Thanks for the patch, I will take care of it.