A different use case I have experienced on non-language wikis (metawiki, mediawikiwiki) was the noise in the results created by listed translation (sub)pages.

A possible partial workaround is to use the intitle: regex feature to exclude titles with a slash in them.

So, Template:PD Help Page gets 184 results on MediaWiki, but Template:PD Help Page -intitle:/\/./ only gets 4.

For some reason -intitle:/\//, which should be "doesn't contain a slash" generates a syntax error. Adding the extra period, which matches anything, is acceptable syntax.

This approach isn't perfect, and it can fail in at least two ways:

• Depending on the wiki and the size of the index, the regex portion could timeout and you could get incomplete results. As with any regex search, including non-regex parts that greatly reduce the amount of text that needs to be scanned by the regex is the way to go. intitle: regexes are less likely to timeout than insource: regexes, and MediaWiki regexes are less likely to timeout than, say, enwiki regexes.
• This won't give the desired results if there is a redirect with a slash in it—which is rare, but possible. For example, the page Extension:FormelApplet has a redirect, Extension:FormelApplet/1.0n, and so the query Extension:FormelApplet -intitle:/\/./ returns 0 results.

Despite these limitations—and depending on your use case—this approach may be of immediate help.

The naming convention for such subpages is different per wiki, e.g. /Doku in dewiki.

TemplateData already has the message MediaWiki:Templatedata-doc-subpage which is customisable per wiki, so if you wish to exclude doc pages with intitle, that is one solution (although it is still not ideal, since in some projects template names can probably contain the generic name for doc subpage).

Also, some projects might have widely used templates in subpages, although that is not the best naming strategy. Therefore I’d actually say that TemplateData, more generally, lacks a way to say which templates are for use in main space and which are not. But that is out of scope for this task.