Problem
There isn't a way to specify that a task does not necessarily represent something actionable. For instance, an Epic is a collection of tasks that need to be worked on for the epic to be resolved. However, an idea is just an idea, it can be rejected or embraced and converted into an epic.
Proposed Solution
Create a generic idea tag for ideas that are in Phabricator.
Alternatively, we could say that ideas should not be on Phabricator, if this is the case, where should they be?
I don't see a need for this. Do you have an actual specific example / use case?
IMHO, most tasks are an idea and [if a report is waiting for further input (e.g. from its reporter or a third party) and can currently not be acted on, the Stalled status is temporarily given.](https://www.mediawiki.org/wiki/Bug_management/Bug_report_life_cycle)
Which underlying problem gets solved by being able to search for such "ideas" across all code repositories via such a new tag? (If that is the expectation, don't know - it's still unclear to me. But if that is the idea, who to add such a tag consistently across all new and old tasks?)
There is Proposal which has been used from time to time to gather comments leading up to a potential Epic, Goal or TechCom-RFC.
Beyond that, I note that at a global level, tags are (in my experience) very rarely used for discovery or search. In part because there is no obvious bound on the scope.
So.. not having a tag at that level might also suffice. I find people gravitate more around component workboards of the actual products and features. I'd personally focus more on having the respective product or team workboard have an intuitive place to find and categorise these.