As suggested by Simetrical on T9804: Namespace filter in Special:Linksearch, I'm opening a new bug to request the namespace filter to be activated for Special:Linksearch on Wikimedia wikis. As far as I understand, this feature needs to be more efficient to be activated (see T9804#125213). Thanks.
- T29717: API's exturlusage module does not respect $wgMiserMode
- T120761: Improve Special:LinkSearch (Community Wishlist 2015, which lists this bug as 1 of 2 desired improvements)