Change #1116544 had a related patch set uploaded (by Awesome Aasim; author: Awesome Aasim):

[mediawiki/core@master] PermissionManager.php: Enable wikis to unlock sandbox subpages for editing (WIP, not finished yet)

https://gerrit.wikimedia.org/r/1116544

This would allow for the proposal and testing of MediaWiki interface messages before they are ultimately created. It would also allow "sandbox" messages to be previewed with the URL parameter "?uselang=sandbox".

I feel like that is problematic from a security perspective. In theory we have separated out all the JS messages, but in practise I don't think we do a good enough job at that.

There already are dozens of "sandbox" subpages here. https://en.wikipedia.org/w/index.php?title=Special%3ASearch&limit=500&offset=0&ns8=1&ns9=1&search=Sandbox+intitle%3Asandbox&advancedSearch-current=%7B%22fields%22%3A%7B%22intitle%22%3A%22sandbox%22%7D%7D&searchToken=1n5z6m27hh4vgy1npwyyfk3cq

I do agree that there may be security concerns if language subpages are not properly separated out from JS. For site JS we could maybe lock it down so that only users with (editinterface) can edit site CSS, JS, and Vue pages.

There might be a better way to do this by writing an extension rather than modifying core. Hmm...

There already are dozens of "sandbox" subpages here. https://en.wikipedia.org/w/index.php?title=Special%3ASearch&limit=500&offset=0&ns8=1&ns9=1&search=Sandbox+intitle%3Asandbox&advancedSearch-current=%7B%22fields%22%3A%7B%22intitle%22%3A%22sandbox%22%7D%7D&searchToken=1n5z6m27hh4vgy1npwyyfk3cq

Sure, and they are mostly in the mediawiki_talk namespace.

In T374483#11130846, @Bawolff wrote:

There already are dozens of "sandbox" subpages here. https://en.wikipedia.org/w/index.php?title=Special%3ASearch&limit=500&offset=0&ns8=1&ns9=1&search=Sandbox+intitle%3Asandbox&advancedSearch-current=%7B%22fields%22%3A%7B%22intitle%22%3A%22sandbox%22%7D%7D&searchToken=1n5z6m27hh4vgy1npwyyfk3cq

Sure, and they are mostly in the mediawiki_talk namespace.

Yeah that is true, but only because it is not possible to create a /sandbox subpage without it being protected from editing for everyone except (editinterface).

The point though is to make testing of how different interface messages appear easier for users who do not have this role. Right now the only way for me to kind of preview how interface messages would appear is to use something like the parse API, but that is not feasible for someone lacking the technical knowledge of APIs.

This proposal addresses a very niche need, and while I understand the motivation behind it, I’m not in favor of changing the long-standing interface editing restrictions. Those restrictions exist to protect against vandalism and unintended security issues, and introducing a special exception for sandboxes would add complexity without bringing much value to the broader community.

Change #1116544 abandoned by Awesome Aasim:

[mediawiki/core@master] PermissionManager/MessageCache: Enable wikis to unlock some sandbox subpages for editing (still need to add tests, etc.)

https://gerrit.wikimedia.org/r/1116544

Ok this idea is dead for now. Until the broader community demonstrates that there indeed is a need to have these pages open for some reason, it is not going to happen.

And if it is going to happen, it more likely will be in some sort of extension (I am not as familiar with writing extensions at this moment) rather than a modification to Core.