Hint if you are in need like me to store multiline private key (Open SSH conversion in Putty as only that azure accept) and then use it in Logic App Standard. You must upload it using Azure CLI,but it makes difference if you upload .txt or .pem file. Latter worked.

az keyvault secret set --name "name-Sftp-sshPrivateKey" --vault-name "kv-name" --file "secretfile.txt" uploaded file ok, but Logic App not connected with it to ssh

file extension changed and voila! az keyvault secret set --name "name-Sftp-sshPrivateKey" --vault-name "kv-name" --file "secretfile.pem"