When you use custom opt-out forms, the opt-outs are not registered in Acoustic Campaign reports. If the opt-out link is a tracked hyperlink, you can get a sense of who opted out by looking at the clicks for that link.
You can still see opt-outs for a email with a custom opt-out. This happens when recipients send an abuse notice through their ISP. This is considered an opt-out in the report.
Note: Bounced email addresses are not included.