ZTK bugs are tracked in individual projects (one per package) using Launchpad’s bug tracker at https://bugs.launchpad.net/zopetoolkit.
XXX, FIXME or TODO comments in the source code are not considered bugs and therefore cannot block releases. Consider adding a bug in addition to the comment. If something is really broken and you want to indicate it with XXX, consider instead not checking this into the trunk of a project but into a separate working branch.
On a loose monthly schedule bug days are organized. The developer community gets together on these days to triage and resolve bugs in Zope projects.
Bug days are announced on the zope-dev mailing list (Weekly IRC meeting).
Historically, bugs have been tracked with the Zope 3 project (https://bugs.launchpad.net/zope3).
Even longer ago, Zope 3 used its own bug tracker (which lived at http://www.zope.org/Collectors). Those were imported into the Launchpad bug database and have aliases of the form “zope3dev-NNN” where “NNN” is the old bug number, so they can be retrieved by searching for these on Launchpad.