The link to the Boilerplate extension is not working. It tries to send to https://phabricator.wikimedia.org/r/project/mediawiki/extensions/BoilerPlate/ which results in a 404.
The current location of the Boilerplate extension seems to be https://phabricator.wikimedia.org/diffusion/EBOP/.
I think maybe everything using Template:Git repo is broken. The history of that template shows that it originally worked with a Lua module mapping Gerrit repo paths to Diffusion callsigns. Later the the Lua module was removed from the template with an explanation that remapping had moved to Phabricator itself. Whatever magic that was does not seem to work these days.
Special:Diff/6680353 should fix things here I think. There are a lot more usages at Special:WhatLinksHere/Template:Git_repo that likely need similar changes.