the new version of langprefix
Fragment of a discussion from User talk:Rua
Dammit. One of the prereqs for {{does-template-exist}} is that the template does have to exist. Did you even read the description on that page? It has a huge honking warning about this.
The restriction on #ifexist: is there to protect the wiki. {{does-template-exist}} is a workaround that circumvents that restriction; when you use it, it is your responsibility to make sure you're using it responsibly.
(I knew that it was a mistake for me to create {{does-template-exist}}. We have too many admins who simply don't give a damn about damaging things, as long as they get to have fun while doing it.)