There are many SFTP clients for .net since many years: SecureBlackbox, IPWorks SSH, WodSFTP, and Rebex SFTP.
SecureBlackbox provides an sftp solution for at least 12 years.
If you are looking for an open source project, SSH.NET on github (https://github.com/sshnet/SSH.NET) should match your expectations.
It is compatible with .net standard witch means it is compatible with .net framework and .net core. Moreover, it is a standalone solution that doesn't have thirdparty dependency.
