I noticed this happening through KDE connect, where passwords copied on Linux show up in Android's clipboard history, is there a way to block passwords from being transported around like that without completely disabling clipboard sharing altogether?

KDE connect lets you disable/configure individual plugins, just disable the "Clipboard sync". I don't think it can by itself figure out that you're copying a password, at least across UI toolkits. FWIW most toolkits and browsers don't actually copy from a password input anyway.

You might want to disable KDE's clipboard history too btw, or at least find a solution that doesn't involve copying passwords. Either use the selection instead of the clipboard, or use password filling through non-clipboard channels.