(Filed by me as the package maintainer.)

The Swiftmailer library has been abandoned upstream since 2021-11. [1]

I consider it unsuitable for release as it no longer receives updates
nor security fixes.

No packages in testing depend on php-swiftmailer anymore.

The official replacement, Symfony Mailer, is available in Debian in
the "php-symfony-mailer" package.

[1] https://github.com/swiftmailer/swiftmailer

As discussed on the mailing list in this thread:
https://alioth-lists.debian.net/pipermail/pkg-php-pear/2025-September/029506.html

I'm planning to take care of php-swiftmailer for the time being to enable
kanboard to enter testing/forky. In parallel, I will investigate moving
kanboard to symfony-mailer or change php-swiftmailer to the FriendsOfSymfony1
fork at https://github.com/FriendsOfSymfony1/swiftmailer.