When your campaign comes to an end, you can avoid broken links by simply disabling donations instead of disabling the campaign.
When donations are disabled, the "Donate" button is removed from your campaign site.
Here's how you can disable donation for your campaign.
Step 1: Navigate to your campaign, and go to Settings > Donation Form.
Step 2: Switch off "Enable Donations" and save.
Now, you can keep your campaign online and no more donations can come through your site!