Not only templates get transcluded on other pages, therefore, in my opinion, this feature should be made available in all common namespaces (not only as an URL parameter). It should probably be default-collapsed on non-template pages.
Version: unspecified
Severity: enhancement
Note that the functionality for this already exists in the code, but it isn't exposed to the user by default. I have a script at https://en.wikipedia.org/wiki/User:Jackmcbarn/advancedtemplatesandbox.js that does expose it.
I wonder if we should add a way to let users use this without needing third-party code.
It seems very likely that displaying TemplateSandbox on most non-template pages would confuse many more users than it would help. As noted in comment 1, a user script already exists for power users to enable this. If someone wants to add this user script to https://www.mediawiki.org/wiki/Extension:TemplateSandbox and/or https://www.mediawiki.org/wiki/Help:Extension:TemplateSandbox, feel free.