For almost 2 years already, the Isabelle project has been using Phabricator quite successfully, in order to stay away from the big black data holes like Bitbucket or Github, see also https://sketis.net/2019/isabelle-phabricator-server-setup
Already in June 2021, the main author and maintainer has announced that the platform is “winding down”, i.e. no longer actively maintained – while a few updates have already appeared afterwards. From a technical standpoint, this somewhat undefined state can easily continue one more year without further worries: our self-hosted server is running well without particular problems.
Moreover, a group of Phabricator users has already started to work on the successor project, which is called “Phorge” and developed on the site we.phorge.it. So lets see how this looks like in 6–12 months, when it is time to reconsider that part of the Isabelle infrastructure.