Clearer documentation would for sure be helpful.
I don't know the use-cases either. Some long, old discussions are here:
But I have not dug through them.
But I don't use that; it's just a (probably wrong) guess.
A hint could be the old wiki page mentioned there:
Now I suspect maybe the original idea was that
project_url can be something else entirely: The "homepage" (which does not necessarily use Trac) of the entire project (which uses Trac for tickets).
(I don't know if this is the actual current or desired configuration for Django, it's just an example.)
In that case project_url should really only be used very rarely, as a link to the project's homepage (possibly unrelated to the project's Trac installation).
Also, in that case maybe
support for project_url can still just be removed. Projects that really want this feature can maybe put it directly in the notification email template?