wholeheartedly agree! Since one can subscribe at different levels to specific tags, categories or sub-categories, this is one of the biggest advantages that discourse has over a mailing list. I’m not sure of all the existing jenkins mailing lists, but I presume that there are several, perhaps there is a sprawl. At a first glance, one might map mailing list to category - but this can also be a mailing list to tag mapping, to reduce the number of categories.
If I remember correctly there is only one level of category nesting - this might have changed since the last time I deployed an instance.
In my experience the thing to consider is how much traffic is being sent to people who subscribe to a particular category or tag, rather than how to organise things. One can always move posts later, re-categorise them etc, but one cannot un-send an email.
I still think the broad categorisation of “user, dev, infra” is a good one, perhaps I’d also decompose “user” into “pipeline author” and “instance admin”.