Board Thread:Support Requests - Getting Technical/@comment-32134737-20200308094108/@comment-1038387-20200308173742

As for the why, that extension is standard on versions 1.31 and higher. Fandom is currently on a forked 1.19, but in the process of upgrading to 1.33. However, there's no guarantee all the available extensions will be used as well.

There may be a suitable alternative on the Dev Wiki. But it won't be a 1:1 copy of TemplateStyles, so if you don't know what it does, it's to just wait it out or consider an alternative.