As it's actually difficult to gather the exact usage of the feature, the best way we have to add deprecation notice in front of users and wait for complains.

Mailing list service of Tuleap rely on tweaked Mailman 2 and with RHEL9* support, this is no longer an option to ship Mailman 2. In the same time, in the last 10 years it became way more complicated to send email (esp. mailing list) because of spam, DMARC & co. Moreover, nowadays most of organization have tools for communication and delivery (Teams and/or Slack most of the time).

The amount of work to have an equivalent of list service based on something else than Mailman 2 is not worth it. We encourage people to use alternative means to send emails.

As start, we should announce removal of Mailman & ForumML for Septembre 2023 (Tuleap 15.0).