On linux (and probably other platforms too), it would be important to mark copied passwords as secrets, so they're properly handled by e.g. clipboard managers and passwords are not exposed by being visibly displayed, stored in unsafe places or included in logs of various sorts.

The way to do so on linux is adding an appropriate mime type. See keepassxc's discussion on this: https://github.com/keepassxreboot/keepassxc/issues/12696

The way keepassxc ended up doing this, is adding the somewhat established "x-kde-passwordManagerHint" mime type, but other mimetypes might be more general, e.g. "text/x-secret" – it wouldn't hurt to include multiple for broader compatibility. This is the current implementation for each platform: https://github.com/keepassxreboot/keepassxc/blob/58aee6f239dc513b6d0e860a5c65d5bd64a54af9/src/gui/Clipboard.cpp#L60

This would be a small thing to implement but would harden proton pass on (at least) Linux quite a bit against accidental password exposure.