Came here to say just that.
If you use a password manager, chances are you don't even know your passwords, so how are you supposed to type them twice? For many websites I am subscribed to I literally never typed my password once, not even in the moment of choosing it.
This is true for most sensitive data, the best option is to have it pre-filled for you via an extension or at least copy paste from another app.
To give a real world example of why disallowing copy paste is inconvenient for the user, just imagine you are on mobile and trying to fill in a form that prevents pasting. You'd have to either split the screen (browser and password manager app) which is always fiddly, or just flip back and forth and copy a password that is probably (and hopefully) not very readable, or manually write it down on a piece of paper and then input it in the form...
How many people would willingly go through this?
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