5.0.6 Stripping '[' and ']' from URLs

Is this for Quick Redirects or regular redirects?

This is for Quick Redirects.

The issue is that you are using sanitize_text_field to clean values, which is removing completely valid characters. I think that this was added to prevent people from putting in potentially harmful characters, but square brackets (especially encoded) should be a valid character.

$destination = sanitize_text_field(trim($data['destination'][$i]));