Even if we have projects for #MediaWiki-Templates and #VisualEditor-MediaWiki-Templates, these projects refer to the software and infrastructure needed to render templates, not to tasks about the templates themselves.
There is a specific audience of developers of Lua or wikitext templates, and there are some tasks that are related with problems in templates (i.e. T101358). Maybe if we would have a #Templates tag, we would connect better existing problems with existing skills? Currently it is basically impossible watch template-related progress in systematic way, since the related tasks might appear anywhere.
I'm personally interested in making Phabricator a better place for template developers, and in improving their developer experience in general. This tag alone will not solve much, but it might help a bit progressing in the right direction (i.e. if a critical mass of template developers join that tag).
PS: and yes, the list of "template" related projects might be confusing for users (see T93546 for even another candidate that might add confusion), but I think this can be fixed with better names / descriptions for all?