You can get then queries like: https://trac.videolan.org/vlc/query?status=assigned&status=n...
And no, tags are not enough.
We might start to look into this again after we've shipped better issue filtering, so that custom fields can actually be used effectively. [1]
[0]: https://gitlab.com/gitlab-org/gitlab-ce/issues/8988
[1]: https://gitlab.com/gitlab-org/gitlab-ce/issues/21993
I'm not surprised, but for us, it's mandatory to replace trac (or bugzilla)
Even better if you have examples of useful fields you've used!