Just figured this one out myself, it's actually surprisingly easy!
Go to Settings > General Management > Language and Input > Autofill Service > None
No longer will it prompt you to use Samsung Pass when logging into a new app!
That will only work in "new apps". To shut down the darn Samsung Pass entirely, go to settings->accounts and remove Samsung account from your phone. You will still have bixby to drive you crazy, even if you remap the button... God, I miss Nokia.
You guys realize this only disables asking you to save the passwords, according to policy and privacy conditions actually Samsung can store our passwords for our interest. We all agreed on this. Just wanted to highlight.
Thank you... this worked... for Android v9.0 you have to do your steps plus Language and Input. The turn off auto input.
As an FYI - this annoying pop-up, Pass asking to Save credentials, just started happening out of the Blue; I disabled and uninstalled anything to do with Samsung Pass and it was still occurring, until I read your post.
So Samsung is shoving another app into the phone. I'm getting tired of that pop up trying to get me to use Samsung pass when I'm trying to log into something with a user name and password. I'm never going to use this dumb program. Anyone got a way to turn it off permanently?