We currently have All-and-every-Wiktionary All-and-every-Wikisource Commons Wikidata.org MediaWiki-extensions-WikimediaIncubator [effectively a tag for Incubator, since it's the only place this extension is installed] wikitech.wikimedia.org

I do not want to create tags for the sake of having tags (and the noise of adding them, and somebody has to do the work to add them, and somebody has to care about what's being tagged), so I am very reluctant.

Then we would be able to gather tasks into a work-board to help everyone do technical work for a particular wiki.

Can you please come up with specific examples to allow clarifying the scope? I'd be interested to understand the problem you would like to solve.
I doubt that "just" creating such tags will help everyone do technical work: Most extensions are deployed on all sites (so would we have to throw 14 site tags on a task just because it happens on all sites?) and only a few extensions are site specific. With regard to gadgets, I'm happy to host projects for specific gadgets if their maintainers want to use Phabricator and make it clear to go here.

To not repeat myself, please see T802 and T78482 for previous discussion. Quoting relevant items here:

In T802#795889, Qgil wrote:

The two main risks we want to avoid are

• Tags of Wikimedia project families being created here without any involvement and awareness of the projects affected, probably leading to tags not being watched and maintained.
• Users reporting tasks under i.e. a "Wikipedia" project when they are not Wikipedia specific, only because they found the problem in Wikipedia. This problem aggravates when there are no active watchers cleaning up.

In T802#936176, @Aklapper wrote:

I'd want clear scope definitions for such tags. "happens on Wikisource" is not clear enough. Tagging every ticket in the Extension-FooBar project with a "Wikisource" tag just because Extension-FooBar is installed on Wikisource will only bring noise.

some extension bugs, while extension is installed everywhere, are project specific

for example, wikisource: T100139 T99440 T90510 T71772 T122712 T128298 T128840 ... (more can be found in a simple search for 'wikisource' in the list of open tasks)

I agree, however to me that's no reason to create tags for all Wikimedia projects. :)

How many project X bugs need to exist before we make a new tag? For example at wiki a category is created if there is at least 5 pages for it. What are the criteria here? Please help me by walking me through the process instead of saying 'there is no reason to'. Thanks.

How many project X bugs need to exist before we make a new tag?
What are the criteria here?

The criteria to create a specific tag is a convincing use case that you have for a specific tag - how exactly the tag is supposed to get used and who plans to use the tag.

Conversation continued on IRC: http://bots.wmflabs.org/~wm-bot/logs/%23mediawiki/20170130.txt

See also T93486

In T154549#2915734, @TTO wrote:

We currently have All-and-every-Wiktionary All-and-every-Wikisource Commons Wikidata.org MediaWiki-extensions-WikimediaIncubator [effectively a tag for Incubator, since it's the only place this extension is installed] wikitech.wikimedia.org

Getting these tags took many years. I filed T40994: Implement a way to index Phabricator tasks by Wikimedia wiki family in August 2012, in the days of Bugzilla, and even then it was not a new idea. :-)

The concerns raised with having wiki family tags have been that tagging can be imperfect (some tasks fall into multiple overlapping categories), there's a maintenance cost associated with each tag, and the tags may go unused by some people in the affected communities. Personally, I think all of these arguments are pretty silly.

On the other hand, I find Phabricator's tagging system to be pretty mediocre and don't really use it much. And any user, by mentioning a term in a task description or in a task comment, can search for that task in Phabricator. Assuming Phabricator search is working. The obvious drawback, of course, is that casual mentions of a project such as Wikidata or Wiktionary, as well as other edge cases, are then included in search results. Using Phabricator tags/projects would be deliberate and explicit as opposed to potentially incidental word association.

Having a tag for "wikibook" would indeed be useful.

Done for wikibooks, see https://phabricator.wikimedia.org/T227158