This is an interesting proposal. Very often some of us are copypasting templates from major Wikipedias (mainly from enwiki) and importing them to local wikis. This is true also for Scribunto modules and gadget scripts (.js and .css). But this brings the issue of minor or less active wikimedia wikis who doesn't have users (or enough users) with enough knowledge to maintain these source codes, and therefore these copies of the original templates soon or later become outdated in regard of the original source codes, bringing also several issues such as less functionalities, deprecated technologies methods or security issues. Also developers of the tools often aren't aware about the copies of their tools in projects other than the developer is maintaining the tool to do the maintenance, or is unable to communicate due to language barrier.
There were a lot of talks about creating a global repository of mediawiki gadgets, either in Commons (I don't know if it's the proper place to store this kind of code) or in Mediawiki. Regarding templates and Scribunto lua modules, would be necessary to create a technical solution to mediawiki core to be able to load a global template in a central wiki with global templates, like this is possible now with media files in Commons. And make a (local) policy requiring developers to enable localisation of the tools. By now I'm not support neither oppose this proposal, and I'll wait for the related Phabricator task to be completed. --Zerabat (discusión) 02:09, 1 February 2018 (UTC)
I agree that it's very interesting and could be very valuable to have, particularly for smaller projects and language versions. Also unsure as to whether a dedicated project would be necessary, or it could be done through an existing one. Like with Commons files, individual wikis would need to have the option to overwrite global templates with local versions to prevent abuse. --BegbertBiggs (talk) 11:00, 5 April 2018 (UTC)
The project description should be changed so that instead of "... used on other Wikimedia Wikis." it reads "... used on other MediaWiki wikis." MediaWiki is available to 3rd parties, Wikitemplates should complement MediaWiki and not be closed only for the use of WMF. --Rob Kam (talk) 15:04, 21 October 2019 (UTC)
New, more detailed proposal
There's a new, and much more detailed proposal in the same spirit as the one this page: mw:Global templates.
Tagging people who supported this one, and may want to take a look at the new one: Lojbanist, Thibdx, Фред-Продавец звёзд, Arepticous, Capankajsmilyo, MJL, Rubbish computer, Moonythedwarf, 沈澄心, Kostas20142, NGC 54, Kitabc12345.