You can now share document(s) and/or folder(s) stored in SharePoint Online to anyone by setting up a password.
This mean even if the link is shared with somebody else, it will not be possible to access the content without the password.
This does not conflict with the other option at your disposal to set an expiration date.
The share with password option is only available when sharing with Anyone (which means you need to have your SharePoint Online administrators to set the sharing option to allow external sharing with anyone).
You will then have to share the defined password using a secure manner (obviously avoid use of email or including it into your custom message)
There is option available for administrators to define some sort of password policy (complexity or banned password) but as you can see in the screenshot above, end-users are notified if the password is weak or strong.
Then the person(s) you shared with will receive notification (as usual) and it will let them know the link can be accessed only with password
