There have been some discussion here and there about the migration of the Zero team to Phabricator. They are using Trello currently.
One additional aspect to consider is that some of their tasks need to be private, since they are dealing with partner information. One theoretical option is to move all the public development to Phabricator, leaving a Trello board for private tasks. For what is worth, Fundraising-Tech have a similar problem, and a solution is being discussed at T88762: Enable select Fundraising people to modify policy for tasks.