If Discourse adds an unsubscribe link in the footer, it’ll also be in a mail header which is automatically surfaced by some mail clients (such as Gmail) as an easily visible Unsubscribe link at the top:
List-Unsubscribe: <https://meta.discourse.org/email/unsubscribe/abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyabcdefghijklm>
List-Unsubscribe-Post: List-Unsubscribe=One-Click
…
To unsubscribe from these emails, [click here](https://meta.discourse.org/email/unsubscribe/abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyabcdefghijklm).
But for OP’s concern, @tobiaseigen is on the mark here:
This means your email provider is adding that List-Unsubscribe header to your outgoing email. The easiest way to sort this out is to look at the account confirmation email and find the header, then report back its contents (redacting the actual token as I’ve done above).