Cannot be closed source unless the maintainer dual licenses and stops making OS (assuming a single maintainer or group that all agree, or that other contributors have signed a relevant copyright waiver).
(though what has already been released can't be retracted)
It has that in common with essentially all other open source license. And it only extends to the version given to you under the license. The copyright holder(s) retain the right to change their mind. Which is a reason AGPL v3 is popular with companies that are looking to sell commercial licenses for their software. There are quite a few startups out there that attempted to keep control whose primary business model is providing a way out from the many restrictions the AGPL v3 imposes on users by offering a commercial license.
In this case, it looks like Plane is not requiring copyright transfers. So that cuts off the practicality of them ever re-licensing their software (commercially or otherwise). That's a good thing as it means it is indeed not likely the license will ever change. The bad thing is that it would be hard to build a company around this project. The AGPLv3 is just very strict about having to open source any and all bundled features and imposes a lot of restrictions and requirements.
What if a project has a plugin architecture and plugins are compiled files that can be dropped in at runtime? Those plugins can be any license then, is it not?
Plug-ins are generally considered a derived work, if directly written to use a specific API. You would need to build a module that works for many different systems and only uses the Plane APIs as a shim.
Even then, the shim might be argued to be infected, and thus the rest of the module too. In that case, you still could be facing an expensive and risky lawsuit, because I don’t think there are any precedents to predict how those winds will blow.
No. Those plugins call functions from AGPL's codebase and they are designed only to be combined with AGPL's work into a larger work, so they are derivative works.
But there is a chance that CSS and svg icons are not required to be GPL.
https://github.com/makeplane/plane/blob/develop/LICENSE.txt