There is no actual fix for this that I know of.
Even if you have turned a live Mailto: link in your signature to plain text, the problem still exists.
This new "feature" (sic) pops up every time you try to send a message and Mail.app finds any email address in the message body or signature that's not already a recipient address in the To:, Cc:, or Bcc: fields.
If you mention someone's email address in the body, boom. If you quote an email chain that has a dozen addresses showing in the text, boom. Mail.app will offer to add (all of?) those addresses as recipients.
It's annoying and disruptive, and we need to be able to switch it off.
Sending Apple feedback now.
…Bob