Current behavior before PR:
When a lead or res.partner click in the 'unsubscribe' link in a mail that was sent to him via the mass mailing module (not via a particular mailing list), he is blacklisted from the whole mass mailing system. The only message he gets is "you have been successfully unsubscribed".
Desired behavior after PR is merged:
The user is told that he was unsubscribed, but in addition a warning is displayed telling him explicitly :
- to which mailing lists he was subscribed
- that he won't receive any mail from them anymore.
Purpose : Warn the user that unsubscribe via a lead or res.partner mass mailing that he won't receive any mail from the mailing lists (listed in the warning) he had subscribed, if any.
closesodoo/odoo#27678