I’m not positive I understand your question, but if you are asking if when I delete a file via SFTP with a tool like FileZilla am I prompted to login, I suppose the answer is yes, once, then I am able to delete anything I want until I close the connection.
I mean like with other built-in WordPress methods. Like if you try to install a plugin via FTP, if you can, I imagine it’d behave similarly.
What I don’t understand is why the plugin (since it has clearly saved all my SSH info) is showing me the screen to enter all my info again when I perform an action that uses the plugin (like deleting a theme). It seems like it’s just confirming my info every time, which is unnecessary.
That’s probably your browser that’s saving the info – not the plugin. The plugin doesn’t save your info anywhere since doing so could constitute a vulnerability. ie. what if WordPress had an SQL injection vulnerability. Just SQL inject a SELECT and you get the SFTP credentials from the DB and then that SQL injection is the least of your concerns.