I've read that post of GSuite Updates google Turning off less secure app access to G Suite accounts where it's commented that after june 2020 the access from LSAs (less secure apps) to GSuite will be limited and I'm not sure that it affects the availability of send emails using smtpClient of c# and only user and password.
I know that it's important to do anyway, but I could plan it later.
Sorry, the explicit question is: That limitation affects the availability to send emails using SmtpClient library of .NET?
Yes; as the documentation says; this means programmatic access via username & password.
You will need to use OAuth instead, which SmtpClient does not support.