When you send a proposal to your clients, you can now add an expiry date to them. After the proposal expires, it will be changed to an "expired" status that will behave similarly to "rejected" proposals in that it will show CTAs to resend the proposal or make edits.
To set an Expiration Date navigate to the Overview of the proposal and click on 'Set expiration', as shown below:
Upon clicking the button, you can pick an expiry date from the calendar selection.
After the expiry, your client will not be able to take an action (Accept or Reject) on the Proposal and you'd need to resend it.