We have progressed by porting a lot of code to use BouncyCastle, which is .NET Core compatible, but unfortunately it won't be finished this year. It is becoming increasingly urgent though, as we're being asked more and more, so we'll have to do it next year.
Our SFTP functionality should be easier to port than FTPS (due to our FTPS implementation's use of Windows certificate APIs), so we're considering releasing the SFTP functionality as .NET Core first. Which protocols are you using?