It was limited to the main namespace since its introduction in
Ieabe8f7071696cde6afbdc6df853aacdb741a4a3. Unfortunately that
commit does not explain the reason.
It should be shown e.g. on user pages (so that sandbox pages
look like the real article) or on project pages (many policy
or help pages on Wikipedia are long and would benefit from it).
It looks like some of the CSS code already assumed that the
TOC would be shown in all namespaces (space for it was reserved
using a 'visibility: hidden;' element on all pages).
Bug: T205312
Change-Id: Id6935f5a7a3701c1c7a38fb37b48b6a3bbc80393
a773c0eed2